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ContentsArticles

Particle physics 1Standard Model 6Supersymmetry 19Elementary particle 26Boson 32Higgs boson 35Higgs mechanism 43Large Hadron Collider 52Peter Higgs 65

Appendix 69

List of particles 69

ReferencesArticle Sources and Contributors 76Image Sources, Licenses and Contributors 79

Article LicensesLicense 80

Particle physics 1

Particle physics

Collision of 2 beams of gold atoms recorded by RHIC

Particle physics is a branch of physics that studies theexistence and interactions of particles that are theconstituents of what is usually referred to as matter orradiation. In current understanding, particles areexcitations of quantum fields and interact followingtheir dynamics. Most of the interest in this area is infundamental fields, each of which cannot be describedas a bound state of other fields. The current set offundamental fields and their dynamics are summarizedin a theory called the Standard Model, thereforeparticle physics is largely the study of the StandardModel's particle content and its possible extensions.

Subatomic particles

The Standard Model of Physics.

Modern particle physics research is focusedon subatomic particles, including atomicconstituents such as electrons, protons, andneutrons (protons and neutrons arecomposite particles called baryons, made ofquarks), particles produced by radioactiveand scattering processes, such as photons,neutrinos, and muons, as well as a widerange of exotic particles. To be specific, theterm particle is a misnomer from classicalphysics because the dynamics of particlephysics are governed by quantummechanics. As such, they exhibitwave-particle duality, displayingparticle-like behavior under certainexperimental conditions and wave-likebehavior in others. In more technical terms,they are described by quantum state vectorsin a Hilbert space, which is also treated inquantum field theory. Following theconvention of particle physicists, elementaryparticles refer to objects such as electronsand photons as it is well known that these types of particles display wave-like properties as well.

All particles and their interactions observed to date can be described almost entirely by a quantum field theory called the Standard Model. The Standard Model has 17 species of elementary particles: 12 fermions or 24 if distinguishing antiparticles, 4 vector bosons (5 with antiparticles), and 1 scalar boson. These elementary particles can combine to

Particle physics 2

form composite particles, accounting for the hundreds of other species of particles discovered since the 1960s. TheStandard Model has been found to agree with almost all the experimental tests conducted to date. However, mostparticle physicists believe that it is an incomplete description of nature, and that a more fundamental theory awaitsdiscovery (See Theory of Everything). In recent years, measurements of neutrino mass have provided the firstexperimental deviations from the Standard Model.Particle physics has impacted the philosophy of science greatly. Some particle physicists adhere to reductionism, apoint of view that has been criticized and defended by philosophers and scientists.[1] [2] [3] [4] Other physicists maydefend the philosophy of holism, which has commonly been viewed to be reductionism's opposite.[5]

HistoryThe idea that all matter is composed of elementary particles dates to at least the 6th century BC. The philosophicaldoctrine of atomism and the nature of elementary particles were studied by ancient Greek philosophers such asLeucippus, Democritus and Epicurus; ancient Indian philosophers such as Kanada, Dignāga and Dharmakirti;medieval scientists such as Alhazen, Avicenna, and Algazel; and early modern European physicists such as PierreGassendi, Robert Boyle, and Isaac Newton. The particle theory of light was also proposed by Alhazen, Avicenna,Gassendi, and Newton. These early ideas were founded in abstract, philosophical reasoning rather thanexperimentation and empirical observation.In the 19th century, John Dalton, through his work on stoichiometry, concluded that each element of nature wascomposed of a single, unique type of particle. Dalton and his contemporaries believed these were the fundamentalparticles of nature and thus named them atoms, after the Greek word atomos, meaning "indivisible". However, nearthe end of the century, physicists discovered that atoms are not, in fact, the fundamental particles of nature, butconglomerates of even smaller particles. The early 20th-century explorations of nuclear physics and quantum physicsculminated in proofs of nuclear fission in 1939 by Lise Meitner (based on experiments by Otto Hahn), and nuclearfusion by Hans Bethe in the same year. These discoveries gave rise to an active industry of generating one atom fromanother, even rendering possible (although not profitable) the transmutation of lead into gold (alchemy). They alsoled to the development of nuclear weapons. Throughout the 1950s and 1960s, a bewildering variety of particles werefound in scattering experiments. This was referred to as the "particle zoo". This term was deprecated after theformulation of the Standard Model during the 1970s in which the large number of particles was explained ascombinations of a (relatively) small number of fundamental particles.

The Standard ModelThe very current state of the classification of elementary particles is the Standard Model. It describes the strong,weak, and electromagnetic fundamental interactions, using mediating gauge bosons. The species of gauge bosons arethe gluons, W−, W+ and Z bosons, and the photons. The model also contains 24 fundamental particles, which are theconstituents of matter. Finally, it predicts the existence of a type of boson known as the Higgs boson, which is yet tobe discovered.

Particle physics 3

Experimental laboratoriesIn particle physics, the major international laboratories are:• Brookhaven National Laboratory (Long Island, United States). Its main facility is the Relativistic Heavy Ion

Collider (RHIC), which collides heavy ions such as gold ions and polarized protons. It is the world's first heavyion collider, and the world's only polarized proton collider.

• Budker Institute of Nuclear Physics (Novosibirsk, Russia). Its main projects are now the electron-positroncolliders VEPP-2000 [6], operated since 2006, and VEPP-4 [7], started experiments in 1994. Earlier facilitiesinclude the first electron-electron beam-beam collider VEP-1, which conducted experiments from 1964 to 1968;the electron-positron colliders VEPP-2, operated from 1965 to 1974; and its successor VEPP-2M [8], performedexperiments in 1974-2000.

• CERN, (Franco-Swiss border, near Geneva). Its main project is now the Large Hadron Collider (LHC), which hadits first beam circulation on 10 September 2008, and is now the world's most energetic collider of protons. It willalso be the most energetic collider of heavy ions when it begins colliding lead ions in 2010. Earlier facilitiesinclude the Large Electron–Positron Collider (LEP), which was stopped in 2001 and then dismantled to give wayfor LHC; and the Super Proton Synchrotron, which is being reused as a pre-accelerator for LHC.

• DESY (Hamburg, Germany). Its main facility is the Hadron Elektron Ring Anlage (HERA), which collideselectrons and positrons with protons.

• Fermilab, (Batavia, United States). Its main facility is the Tevatron, which collides protons and antiprotons andwas the highest-energy particle collider in the world until the Large Hadron Collider surpassed it on 29 November2009.

• KEK, (Tsukuba, Japan). It is the home of a number of experiments such as the K2K experiment, a neutrinooscillation experiment and Belle, an experiment measuring the CP violation of B mesons.

Many other particle accelerators exist.The techniques required to do modern experimental particle physics are quite varied and complex, constituting asub-specialty nearly completely distinct from the theoretical side of the field.

TheoryTheoretical particle physics attempts to develop the models, theoretical framework, and mathematical tools tounderstand current experiments and make predictions for future experiments. See also theoretical physics. There areseveral major interrelated efforts in theoretical particle physics today. One important branch attempts to betterunderstand the Standard Model and its tests. By extracting the parameters of the Standard Model from experimentswith less uncertainty, this work probes the limits of the Standard Model and therefore expands our understanding ofnature's building blocks. These efforts are made challenging by the difficulty of calculating quantities in quantumchromodynamics. Some theorists working in this area refer to themselves as phenomenologists and may use thetools of quantum field theory and effective field theory. Others make use of lattice field theory and call themselveslattice theorists.Another major effort is in model building where model builders develop ideas for what physics may lie beyond theStandard Model (at higher energies or smaller distances). This work is often motivated by the hierarchy problem andis constrained by existing experimental data. It may involve work on supersymmetry, alternatives to the Higgsmechanism, extra spatial dimensions (such as the Randall-Sundrum models), Preon theory, combinations of these, orother ideas.A third major effort in theoretical particle physics is string theory. String theorists attempt to construct a unifieddescription of quantum mechanics and general relativity by building a theory based on small strings, and branesrather than particles. If the theory is successful, it may be considered a "Theory of Everything".

Particle physics 4

There are also other areas of work in theoretical particle physics ranging from particle cosmology to loop quantumgravity.This division of efforts in particle physics is reflected in the names of categories on the arXiv, a preprint archive [9]:hep-th (theory), hep-ph (phenomenology), hep-ex (experiments), hep-lat (lattice gauge theory).

The futureThe overarching goal, which is pursued in several distinct ways, is to find and understand what physics may liebeyond the standard model. There are several powerful experimental reasons to expect new physics, including darkmatter and neutrino mass. There are also theoretical hints that this new physics should be found at accessible energyscales. Furthermore, there may be unexpected and unpredicted surprises that will give us the most opportunity tolearn about nature.Much of the efforts to find this new physics are focused on new collider experiments. The Large Hadron Collider(LHC) was completed in 2008 to help continue the search for the Higgs boson, supersymmetric particles, and othernew physics. An intermediate goal is the construction of the International Linear Collider (ILC), which willcomplement the LHC by allowing more precise measurements of the properties of newly found particles. In August2004, a decision for the technology of the ILC was taken but the site has still to be agreed upon.In addition, there are important non-collider experiments that also attempt to find and understand physics beyond theStandard Model. One important non-collider effort is the determination of the neutrino masses, since these massesmay arise from neutrinos mixing with very heavy particles. In addition, cosmological observations provide manyuseful constraints on the dark matter, although it may be impossible to determine the exact nature of the dark matterwithout the colliders. Finally, lower bounds on the very long lifetime of the proton put constraints on Grand UnifiedTheories at energy scales much higher than collider experiments will be able to probe any time soon.

Notes[1] "Review of particle physics" (http:/ / pdg. lbl. gov/ ). .[2] "Particle Physics News and Resources" (http:/ / www. interactions. org/ ). .[3] "CERN Courier - International Journal of High-Energy Physics" (http:/ / cerncourier. com). .[4] "Particle physics in 60 seconds" (http:/ / www. symmetrymagazine. org/ cms/ ?pid=1000345). .[5] "Quantum Holism" (http:/ / www. bu. edu/ wcp/ Papers/ Scie/ ScieTsek. htm). .[6] http:/ / vepp2k. inp. nsk. su/[7] http:/ / v4. inp. nsk. su/ index. en. html[8] http:/ / www. inp. nsk. su/ activity/ old/ vepp2m/ index. ru. shtml[9] http:/ / www. arxiv. org

References

Further reading

General readers• Frank Close (2004) Particle Physics: A Very Short Introduction. Oxford University Press. ISBN 0-19-280434-0.• --------, Michael Marten, and Christine Sutton (2002) The Particle Odyssey: A Journey to the Heart of the Matter.

Oxford Univ. Press. ISBN 0-19-850486-1.• Ford, Kenneth W. (2005) The Quantum World. Harvard Univ. Press.• Oerter, Robert (2006) The Theory of Almost Everything: The Standard Model, the Unsung Triumph of Modern

Physics. Plume.• Schumm, Bruce A. (2004) Deep Down Things: The Breathtaking Beauty of Particle Physics. John Hopkins Univ.

Press. ISBN 0-8018-7971-X.

Particle physics 5

• Riazuddin, PhD. "An Overview of Particle Physics and Cosmology" (http:/ / indico. ncp. edu. pk/ indico/ getFile.py/ access?sessionId=0& resId=0& materialId=0& confId=10). NCP Journal of Physics (Dr. Professor Riazuddin,High Energy Theory Group, and senior scientist at the National Center for Nuclear Physics) 1 (1): 50.

Gentle texts• Frank Close (2006) The New Cosmic Onion. Taylor & Francis. ISBN 1-58488-798-2.• Lincoln Wolfenstein & Joao P. Silva (2010) Exploring Fundamental Particles . Taylor & Francis. ISBN

978-143-983-612-5 .• Coughlan, G. D., J. E. Dodd, and B. M. Gripaios (2006) The Ideas of Particle Physics: An Introduction for

Scientists, 3rd ed. Cambridge Univ. Press. An undergraduate text for those not majoring in physics.

HarderA survey article:• Robinson, Matthew B., Gerald Cleaver, and J. R. Dittmann (2008) "A Simple Introduction to Particle Physics" -

Part 1, 135pp. (http:/ / arxiv. org/ abs/ 0810. 3328v1) and Part 2, nnnpp. (http:/ / arxiv. org/ abs/ 0908. 1395v1)Baylor University Dept. of Physics.

Texts:• Griffiths, David J. (1987). Introduction to Elementary Particles. Wiley, John & Sons, Inc. ISBN 0-471-60386-4.• Kane, Gordon L. (1987). Modern Elementary Particle Physics. Perseus Books. ISBN 0-201-11749-5.• Perkins, Donald H. (1999). Introduction to High Energy Physics. Cambridge University Press.

ISBN 0-521-62196-8.• Povh, Bogdan (1995). Particles and Nuclei: An Introduction to the Physical Concepts. Springer-Verlag.

ISBN 0-387-59439-6.• Boyarkin, Oleg (2011). Advanced Particle Physics Two-Volume Set. CRC Press. ISBN 978-143-980-412-4.

External links• The Particle Adventure (http:/ / particleadventure. org/ ) - educational project sponsored by the Particle Data

Group of the Lawrence Berkeley National Laboratory (LBNL)• symmetry magazine (http:/ / www. symmetrymagazine. org)• Particle physics – it matters (http:/ / www. iop. org/ publications/ iop/ 2009/ page_38211. html) - the Institute of

Physics• Nobes, Matthew (2002) "Introduction to the Standard Model of Particle Physics" on Kuro5hin: Part 1, (http:/ /

www. kuro5hin. org/ story/ 2002/ 5/ 1/ 3712/ 31700) Part 2, (http:/ / www. kuro5hin. org/ story/ 2002/ 5/ 14/19363/ 8142) Part 3a, (http:/ / www. kuro5hin. org/ story/ 2002/ 7/ 15/ 173318/ 784) Part 3b. (http:/ / www.kuro5hin. org/ story/ 2002/ 8/ 21/ 195035/ 576)

• CERN (http:/ / public. web. cern. ch/ public/ ) - European Organization for Nuclear Research• Fermilab (http:/ / www. fnal. gov/ )

Standard Model 6

Standard Model

The Standard Model of elementary particles, with the gauge bosons in therightmost column

The Standard Model of particle physics isa theory concerning the electromagnetic,weak, and strong nuclear interactions, whichmediate the dynamics of the knownsubatomic particles. Developed throughoutthe mid to late 20th century, the currentformulation was finalized in the mid 1970supon experimental confirmation of theexistence of quarks. Since then, discoveriesof the bottom quark (1977), the top quark(1995) and the tau neutrino (2000) havegiven credence to the Standard Model.Because of its success in explaining a widevariety of experimental results, the StandardModel is sometimes regarded as a theory ofalmost everything.

Still, the Standard Model falls short of beinga complete theory of fundamentalinteractions because it does not incorporatethe physics of dark energy nor of the fulltheory of gravitation as described by generalrelativity. The theory does not contain anyviable dark matter particle that possesses all of the required properties deduced from observational cosmology. Italso does not correctly account for neutrino oscillations (and their non-zero masses). Although the Standard Model istheoretically self-consistent, it has several apparently unnatural properties giving rise to puzzles like the strong CPproblem and the hierarchy problem.

Nevertheless, the Standard Model is important to theoretical and experimental particle physicists alike. For theorists,the Standard Model is a paradigmatic example of a quantum field theory, which exhibits a wide range of physicsincluding spontaneous symmetry breaking, anomalies, non-perturbative behavior, etc. It is used as a basis forbuilding more exotic models which incorporate hypothetical particles, extra dimensions and elaborate symmetries(such as supersymmetry) in an attempt to explain experimental results at variance with the Standard Model such asthe existence of dark matter and neutrino oscillations. In turn, the experimenters have incorporated the standardmodel into simulators to help search for new physics beyond the Standard Model from relatively uninterestingbackground.Recently, the standard model has found applications in fields besides particle physics, such as astrophysics,cosmology, and nuclear physics.

Standard Model 7

Historical backgroundThe first step towards the Standard Model was Sheldon Glashow's discovery in 1960 of a way to combine theelectromagnetic and weak interactions.[1] In 1967 Steven Weinberg[2] and Abdus Salam[3] incorporated the Higgsmechanism[4] [5] [6] into Glashow's electroweak theory, giving it its modern form.The Higgs mechanism is believed to give rise to the masses of all the elementary particles in the Standard Model.This includes the masses of the W and Z bosons, and the masses of the fermions, i.e. the quarks and leptons.After the neutral weak currents caused by Z boson exchange were discovered at CERN in 1973,[7] [8] [9] [10] theelectroweak theory became widely accepted and Glashow, Salam, and Weinberg shared the 1979 Nobel Prize inPhysics for discovering it. The W and Z bosons were discovered experimentally in 1981, and their masses werefound to be as the Standard Model predicted.The theory of the strong interaction, to which many contributed, acquired its modern form around 1973–74, whenexperiments confirmed that the hadrons were composed of fractionally charged quarks.

OverviewAt present, matter and energy are best understood in terms of the kinematics and interactions of elementary particles.To date, physics has reduced the laws governing the behavior and interaction of all known forms of matter andenergy to a small set of fundamental laws and theories. A major goal of physics is to find the "common ground" thatwould unite all of these theories into one integrated theory of everything, of which all the other known laws wouldbe special cases, and from which the behavior of all matter and energy could be derived (at least in principle).[11]

The Standard Model groups two major extant theories—quantum electroweak and quantum chromodynamics—intoan internally consistent theory that describes the interactions between all known particles in terms of quantum fieldtheory. For a technical description of the fields and their interactions, see Standard Model (mathematicalformulation).

Particle content

Fermions

Organization of Fermions

Charge First generation Second generation Third generation

Quarks +2⁄3Up u Charm c Top t

−1⁄3Down d Strange s Bottom b

Leptons −1 Electron e− Muon μ− Tau τ−

0 Electron neutrino νe Muon neutrino νμ Tau neutrino ντ

The Standard Model includes 12 elementary particles of spin-1⁄2 known as fermions. According to the spin-statisticstheorem, fermions respect the Pauli exclusion principle. Each fermion has a corresponding antiparticle.The fermions of the Standard Model are classified according to how they interact (or equivalently, by what chargesthey carry). There are six quarks (up, down, charm, strange, top, bottom), and six leptons (electron, electronneutrino, muon, muon neutrino, tau, tau neutrino). Pairs from each classification are grouped together to form ageneration, with corresponding particles exhibiting similar physical behavior (see table).The defining property of the quarks is that they carry color charge, and hence, interact via the strong interaction. A phenomenon called color confinement results in quarks being perpetually (or at least since very soon after the start of

Standard Model 8

the Big Bang) bound to one another, forming color-neutral composite particles (hadrons) containing either a quarkand an antiquark (mesons) or three quarks (baryons). The familiar proton and the neutron are the two baryons havingthe smallest mass. Quarks also carry electric charge and weak isospin. Hence they interact with other fermions bothelectromagnetically and via the weak interaction.The remaining six fermions do not carry color charge and are called leptons. The three neutrinos do not carry electriccharge either, so their motion is directly influenced only by the weak nuclear force, which makes them notoriouslydifficult to detect. However, by virtue of carrying an electric charge, the electron, muon, and tau all interactelectromagnetically.Each member of a generation has greater mass than the corresponding particles of lower generations. The firstgeneration charged particles do not decay; hence all ordinary (baryonic) matter is made of such particles.Specifically, all atoms consist of electrons orbiting atomic nuclei ultimately constituted of up and down quarks.Second and third generations charged particles, on the other hand, decay with very short half lives, and are observedonly in very high-energy environments. Neutrinos of all generations also do not decay, and pervade the universe, butrarely interact with baryonic matter.

Gauge bosons

Summary of interactions between particles described by the Standard Model.

In the Standard Model, gauge bosonsare defined as force carriers thatmediate the strong, weak, andelectromagnetic fundamentalinteractions.

Interactions in physics are the waysthat particles influence other particles.At a macroscopic level,electromagnetism allows particles tointeract with one another via electricand magnetic fields, and gravitationallows particles with mass to attractone another in accordance withEinstein's theory of general relativity.The standard model explains suchforces as resulting from matterparticles exchanging other particles, known as force mediating particles (strictly speaking, this is only so ifinterpreting literally what is actually an approximation method known as perturbation theory). When a forcemediating particle is exchanged, at a macroscopic level the effect is equivalent to a force influencing both of them,and the particle is therefore said to have mediated (i.e., been the agent of) that force. The Feynman diagramcalculations, which are a graphical representation of the perturbation theory approximation, invoke "force mediatingparticles", and when applied to analyze high-energy scattering experiments are in reasonable agreement with thedata. However, perturbation theory (and with it the concept of a "force-mediating particle") fails in other situations.These include low-energy quantum chromodynamics, bound states, and solitons.

The gauge bosons of the Standard Model all have spin (as do matter particles). The value of the spin is 1, makingthem bosons. As a result, they do not follow the Pauli exclusion principle that constrains leptons: thus bosons (e.g.photons) do not have a theoretical limit on their spatial density (number per volume). The different types of gaugebosons are described below.

Standard Model 9

• Photons mediate the electromagnetic force between electrically charged particles. The photon is massless and iswell-described by the theory of quantum electrodynamics.

• The W+, W−, and Z gauge bosons mediate the weak interactions between particles of different flavors (all quarksand leptons). They are massive, with the Z being more massive than the W±. The weak interactions involving theW± exclusively act on left-handed particles and right-handed antiparticles only. Furthermore, the W± carries anelectric charge of +1 and −1 and couples to the electromagnetic interaction. The electrically neutral Z bosoninteracts with both left-handed particles and antiparticles. These three gauge bosons along with the photons aregrouped together, as collectively mediating the electroweak interaction.

• The eight gluons mediate the strong interactions between color charged particles (the quarks). Gluons aremassless. The eightfold multiplicity of gluons is labeled by a combination of color and anticolor charge (e.g.red–antigreen).[12] Because the gluon has an effective color charge, they can also interact among themselves. Thegluons and their interactions are described by the theory of quantum chromodynamics.

The interactions between all the particles described by the Standard Model are summarized by the diagram at the topof this section.

Higgs bosonThe Higgs particle is a hypothetical massive scalar elementary particle theorized by Robert Brout, François Englert,Peter Higgs, Gerald Guralnik, C. R. Hagen, and Tom Kibble in 1964 (see 1964 PRL symmetry breaking papers) andis a key building block in the Standard Model.[13] [14] [15] [16] It has no intrinsic spin, and for that reason is classifiedas a boson (like the gauge bosons, which have integer spin). Because an exceptionally large amount of energy andbeam luminosity are theoretically required to observe a Higgs boson in high energy colliders, it is the onlyfundamental particle predicted by the Standard Model that has yet to be observed.The Higgs boson plays a unique role in the Standard Model, by explaining why the other elementary particles,except the photon and gluon, are massive. In particular, the Higgs boson would explain why the photon has no mass,while the W and Z bosons are very heavy. Elementary particle masses, and the differences betweenelectromagnetism (mediated by the photon) and the weak force (mediated by the W and Z bosons), are critical tomany aspects of the structure of microscopic (and hence macroscopic) matter. In electroweak theory, the Higgsboson generates the masses of the leptons (electron, muon, and tau) and quarks.As yet, no experiment has conclusively detected the existence of the Higgs boson. It is hoped that the Large HadronCollider at CERN will confirm the existence of this particle. As of August 2011, a significant portion of the possiblemasses for the Higgs have been excluded at 95% confidence level: CMS has excluded the mass ranges 145-216GeV, 226-288 GeV and 310-400 GeV,[17] while the ATLAS experiment has excluded 146-232 GeV, 256-282 GeVand 296-466 GeV.[18] Note that these exclusions apply only to the Standard Model Higgs, and that more complexHiggs sectors which are possible in Beyond the Standard Model scenarios may be significantly more difficult tocharacterize. CERN director general Rolf Heuer has predicted that by the end of 2012 either the Standard ModelHiggs boson will be observed, or excluded in all mass ranges, implying that the Standard Model is not the wholestory.[19]

Standard Model 10

Field contentThe standard model has the following fields:

Spin 11. A U(1) gauge field Bμν with coupling g′ (weak U(1), or weak hypercharge)2. An SU(2) gauge field Wμν with coupling g (weak SU(2), or weak isospin)3. An SU(3) gauge field Gμν with coupling gs (strong SU(3), or color charge)

Spin 1⁄2The spin 1⁄2 particles are in representations of the gauge groups. For the U(1) group, we list the value of the weakhypercharge instead. The left-handed fermionic fields are:1. An SU(3) triplet, SU(2) doublet, with U(1) weak hypercharge 1⁄3 (left-handed quarks)2. An SU(3) triplet, SU(2) singlet, with U(1) weak hypercharge 2⁄3 (left-handed down-type antiquark)3. An SU(3) singlet, SU(2) doublet with U(1) weak hypercharge −1 (left-handed lepton)4. An SU(3) triplet, SU(2) singlet, with U(1) weak hypercharge −4⁄3 (left-handed up-type antiquark)5. An SU(3) singlet, SU(2) singlet with U(1) weak hypercharge 2 (left-handed antilepton)By CPT symmetry, there is a set of right-handed fermions with the opposite quantum numbers.This describes one generation of leptons and quarks, and there are three generations, so there are three copies of eachfield. Note that there are twice as many left-handed lepton field components as left-handed antilepton fieldcomponents in each generation, but an equal number of left-handed quark and antiquark fields.

Spin 01. An SU(2) doublet H with U(1) hyper-charge −1 (Higgs field)Note that |H|2, summed over the two SU(2) components, is invariant under both SU(2) and under U(1), and so it canappear as a renormalizable term in the Lagrangian, as can its square.This field acquires a vacuum expectation value, leaving a combination of the weak isospin, I3, and weak hyperchargeunbroken. This is the electromagnetic gauge group, and the photon remains massless. The standard formula for theelectric charge (which defines the normalization of the weak hypercharge, Y, which would otherwise be somewhatarbitrary) is:[20]

LagrangianThe Lagrangian for the spin 1 and spin 1⁄2 fields is the most general renormalizable gauge field Lagrangian with nofine tunings:• Spin 1:

where the traces are over the SU(2) and SU(3) indices hidden in W and G respectively. The two-index objects are thefield strengths derived from W and G the vector fields. There are also two extra hidden parameters: the theta anglesfor SU(2) and SU(3).The spin-1⁄2 particles can have no mass terms because there is no right/left helicity pair with the same SU(2) andSU(3) representation and the same weak hypercharge. This means that if the gauge charges were conserved in thevacuum, none of the spin 1⁄2 particles could ever swap helicity, and they would all be massless.

Standard Model 11

For a neutral fermion, for example a hypothetical right-handed lepton N (or Nα in relativistic two-spinor notation),with no SU(3), SU(2) representation and zero charge, it is possible to add the term:

This term gives the neutral fermion a Majorana mass. Since the generic value for M will be of order 1, such a particlewould generically be unacceptably heavy. The interactions are completely determined by the theory – the leptonsintroduce no extra parameters.

Higgs mechanismThe Lagrangian for the Higgs includes the most general renormalizable self interaction:

The parameter v2 has dimensions of mass squared, and it gives the location where the classical Lagrangian is at aminimum. In order for the Higgs mechanism to work, v2 must be a positive number. v has units of mass, and it is theonly parameter in the standard model which is not dimensionless. It is also much smaller than the Planck scale; it isapproximately equal to the Higgs mass, and sets the scale for the mass of everything else. This is the only realfine-tuning to a small nonzero value in the standard model, and it is called the Hierarchy problem.It is traditional to choose the SU(2) gauge so that the Higgs doublet in the vacuum has expectation value (v,0).

Masses and CKM matrixThe rest of the interactions are the most general spin-0 spin-1⁄2 Yukawa interactions, and there are many of these.These constitute most of the free parameters in the model. The Yukawa couplings generate the masses and mixingsonce the Higgs gets its vacuum expectation value.The terms L*HR generate a mass term for each of the three generations of leptons. There are 9 of these terms, but byrelabeling L and R, the matrix can be diagonalized. Since only the upper component of H is nonzero, the upperSU(2) component of L mixes with R to make the electron, the muon, and the tau, leaving over a lower masslesscomponent, the neutrino. Note: Neutrino oscillations show neutrinos have mass.[21] See also:Pontecorvo–Maki–Nakagawa–Sakata matrix.The terms QHU generate up masses, while QHD generate down masses. But since there is more than oneright-handed singlet in each generation, it is not possible to diagonalize both with a good basis for the fields, andthere is an extra CKM matrix.

Theoretical aspects

Construction of the Standard Model Lagrangian

Standard Model 12

Parameters of the Standard Model

Symbol Description Renormalizationscheme (point)

Value

me Electron mass 511 keV

mμ Muon mass 105.7 MeV

mτ Tau mass 1.78 GeV

mu Up quark mass μMS = 2 GeV 1.9 MeV

md Down quark mass μMS = 2 GeV 4.4 MeV

ms Strange quark mass μMS = 2 GeV 87 MeV

mc Charm quark mass μMS = mc 1.32 GeV

mb Bottom quark mass μMS = mb 4.24 GeV

mt Top quark mass On-shell scheme 172.7 GeV

θ12 CKM 12-mixing angle 13.1°

θ23 CKM 23-mixing angle 2.4°

θ13 CKM 13-mixing angle 0.2°

δ CKM CP-violating Phase 0.995

g1 U(1) gauge coupling μMS = mZ 0.357

g2 SU(2) gauge coupling μMS = mZ 0.652

g3 SU(3) gauge coupling μMS = mZ 1.221

θQCD QCD vacuum angle ~0

μ Higgs quadratic coupling Unknown

λ Higgs self-coupling strength Unknown

Technically, quantum field theory provides the mathematical framework for the standard model, in which aLagrangian controls the dynamics and kinematics of the theory. Each kind of particle is described in terms of adynamical field that pervades space-time. The construction of the standard model proceeds following the modernmethod of constructing most field theories: by first postulating a set of symmetries of the system, and then by writingdown the most general renormalizable Lagrangian from its particle (field) content that observes these symmetries.The global Poincaré symmetry is postulated for all relativistic quantum field theories. It consists of the familiartranslational symmetry, rotational symmetry and the inertial reference frame invariance central to the theory ofspecial relativity. The local SU(3)×SU(2)×U(1) gauge symmetry is an internal symmetry that essentially defines thestandard model. Roughly, the three factors of the gauge symmetry give rise to the three fundamental interactions.The fields fall into different representations of the various symmetry groups of the Standard Model (see table). Uponwriting the most general Lagrangian, one finds that the dynamics depend on 19 parameters, whose numerical valuesare established by experiment. The parameters are summarized in the table at right.

Standard Model 13

Quantum chromodynamics sector

The quantum chromodynamics (QCD) sector defines the interactions between quarks and gluons, with SU(3)symmetry, generated by Ta. Since leptons do not interact with gluons, they are not affected by this sector. The DiracLagrangian of the quarks coupled to the gluon fields is given by

is the SU(3) gauge field containing the gluons, are the Dirac matrices, D and U are the Dirac spinorsassociated with up- and down-type quarks, and gs is the strong coupling constant.

Electroweak sector

The electroweak sector is a Yang–Mills gauge theory with the symmetry group U(1)×SU(2)L,

where Bμ is the U(1) gauge field; YW is the weak hypercharge—the generator of the U(1) group; is the

three-component SU(2) gauge field; are the Pauli matrices—infinitesimal generators of the SU(2) group. Thesubscript L indicates that they only act on left fermions; g′ and g are coupling constants.

Higgs sector

In the Standard Model, the Higgs field is a complex spinor of the group SU(2)L:

where the indexes + and 0 indicate the electric charge (Q) of the components. The weak isospin (YW) of bothcomponents is 1.Before symmetry breaking, the Higgs Lagrangian is:

which can also be written as:

Additional symmetries of the Standard ModelFrom the theoretical point of view, the Standard Model exhibits four additional global symmetries, not postulated atthe outset of its construction, collectively denoted accidental symmetries, which are continuous U(1) globalsymmetries. The transformations leaving the Lagrangian invariant are:

The first transformation rule is shorthand meaning that all quark fields for all generations must be rotated by anidentical phase simultaneously. The fields , and , are the 2nd (muon) and 3rd (tau)generation analogs of and fields.By Noether's theorem, each symmetry above has an associated conservation law: the conservation of baryon number, electron number, muon number, and tau number. Each quark is assigned a baryon number of 1/3, while each antiquark is assigned a baryon number of -1/3. Conservation of baryon number implies that the number of quarks

Standard Model 14

minus the number of antiquarks is a constant. Within experimental limits, no violation of this conservation law hasbeen found.Similarly, each electron and its associated neutrino is assigned an electron number of +1, while the anti-electron andthe associated anti-neutrino carry a −1 electron number. Similarly, the muons and their neutrinos are assigned amuon number of +1 and the tau leptons are assigned a tau lepton number of +1. The Standard Model predicts thateach of these three numbers should be conserved separately in a manner similar to the way baryon number isconserved. These numbers are collectively known as lepton family numbers (LF).Symmetry works differently for quarks than for leptons, mainly because the Standard Model predicts (incorrectly)that neutrinos are massless. However, in 2002 it was discovered that neutrinos have mass (now established to be notgreater than 0.28 electron volts), and as neutrinos oscillate between flavors (muon neutrinos have been observedchanging to tau neutrinos) the discovery of neutrino mass indicates that the conservation of lepton family number isviolated.[22]

In addition to the accidental (but exact) symmetries described above, the Standard Model exhibits severalapproximate symmetries. These are the "SU(2) custodial symmetry" and the "SU(2) or SU(3) quark flavorsymmetry."

Symmetries of the Standard Model and Associated Conservation Laws

Symmetry Lie Group Symmetry Type Conservation Law

Poincaré Translations×SO(3,1) Global symmetry Energy, Momentum, Angular momentum

Gauge SU(3)×SU(2)×U(1) Local symmetry Color charge, Weak isospin, Electric charge, Weak hypercharge

Baryon phase U(1) Accidental Global symmetry Baryon number

Electron phase U(1) Accidental Global symmetry Electron number

Muon phase U(1) Accidental Global symmetry Muon number

Tau phase U(1) Accidental Global symmetry Tau number

Field content of the Standard Model

Field(1st generation)

Spin Gauge groupRepresentation

BaryonNumber

ElectronNumber

Left-handed quark ( , , )

Left-handed up antiquark ( , , )

Left-handed down antiquark ( , , )

Left-handed lepton ( , , )

Left-handed antielectron ( , , )

Hypercharge gauge field ( , , )

Isospin gauge field ( , , )

Gluon field ( , , )

Higgs field ( , , )

Standard Model 15

List of standard model fermionsThis table is based in part on data gathered by the Particle Data Group.[23]

Left-handed fermions in the Standard Model

Generation 1

Fermion(left-handed)

Symbol Electriccharge

Weakisospin

Weakhypercharge

Colorcharge *

Mass **

Electron 511 keV

Positron 511 keV

Electron neutrino < 0.28 eV ****

Electron antineutrino < 0.28 eV ****

Up quark ~ 3 MeV ***

Up antiquark ~ 3 MeV ***

Down quark ~ 6 MeV ***

Down antiquark ~ 6 MeV ***

Generation 2

Fermion(left-handed)

Symbol Electriccharge

Weakisospin

Weakhypercharge

Colorcharge *

Mass **

Muon 106 MeV

Antimuon 106 MeV

Muon neutrino < 0.28 eV ****

Muon antineutrino < 0.28 eV ****

Charm quark ~ 1.337 GeV

Charm antiquark ~ 1.3 GeV

Strange quark ~ 100 MeV

Strange antiquark ~ 100 MeV

Generation 3

Fermion(left-handed)

Symbol Electriccharge

Weakisospin

Weakhypercharge

Colorcharge *

Mass **

Tau 1.78 GeV

Antitau 1.78 GeV

Tau neutrino < 0.28 eV ****

Tau antineutrino < 0.28 eV ****

Top quark 171 GeV

Top antiquark 171 GeV

Bottom quark ~ 4.2 GeV

Bottom antiquark ~ 4.2 GeV

Standard Model 16

Notes:

• * These are not ordinary abelian charges, which can be added together, but are labels of group representations of Lie groups.• ** Mass is really a coupling between a left-handed fermion and a right-handed fermion. For example, the mass of an electron is really a

coupling between a left-handed electron and a right-handed electron, which is the antiparticle of a left-handed positron. Also neutrinos showlarge mixings in their mass coupling, so it's not accurate to talk about neutrino masses in the flavor basis or to suggest a left-handed electronantineutrino.

• *** The masses of baryons and hadrons and various cross-sections are the experimentally measured quantities. Since quarks can't be isolatedbecause of QCD confinement, the quantity here is supposed to be the mass of the quark at the renormalization scale of the QCD scale.

• **** The Standard Model assumes that neutrinos are massless. However, several contemporary experiments prove that neutrinos oscillatebetween their flavour states, which could not happen if all were massless.[24] It is straightforward to extend the model to fit these data but thereare many possibilities, so the mass eigenstates are still open. See Neutrino#Mass.

Log plot of masses in the Standard Model.

Tests and predictions

The Standard Model (SM) predictedthe existence of the W and Z bosons,gluon, and the top and charm quarksbefore these particles were observed.Their predicted properties wereexperimentally confirmed with goodprecision. To give an idea of thesuccess of the SM, the following table compares the measured masses of the W and Z bosons with the massespredicted by the SM:

Quantity Measured (GeV) SM prediction (GeV)

Mass of W boson 80.398 ± 0.025 80.390 ± 0.018

Mass of Z boson 91.1876 ± 0.0021 91.1874 ± 0.0021

The SM also makes several predictions about the decay of Z bosons, which have been experimentally confirmed bythe Large Electron-Positron Collider at CERN.

ChallengesThere is some experimental evidence consistent with neutrinos having mass, which the Standard Model does notallow.[25] To accommodate such findings, the Standard Model can be modified by adding a non-renormalizableinteraction of lepton fields with the square of the Higgs field. This is natural in certain grand unified theories, and ifnew physics appears at about 1016 GeV, the neutrino masses are of the right order of magnitude.Currently, there is one elementary particle predicted by the Standard Model that has yet to be observed: the Higgsboson. A major reason for building the Large Hadron Collider is that the high energies of which it is capable areexpected to make the Higgs observable. However, as of January 2011, there is only indirect empirical evidence forthe existence of the Higgs boson, so that its discovery cannot be claimed. Moreover, some theoretical concerns havebeen raised positing that elementary scalar Higgs particles cannot exist (see Quantum triviality).A fair amount of theoretical and experimental research has attempted to extend the Standard Model into a UnifiedField Theory or a Theory of everything, a complete theory explaining all physical phenomena including constants.Inadequacies of the Standard Model that motivate such research include:• It does not attempt to explain gravitation, and unlike for the strong and electroweak interactions of the Standard

Model, there is no known way of describing general relativity, the canonical theory of gravitation, consistently in terms of quantum field theory. The reason for this is among other things that quantum field theories of gravity

Standard Model 17

generally break down before reaching the Planck scale. As a consequence, we have no reliable theory for the veryearly universe;

• It is considered by experts to be ad-hoc and inelegant, requiring 19 numerical constants whose values areunrelated and arbitrary. Although the Standard Model, as it now stands, can explain why neutrinos have masses,the specifics of neutrino mass are still unclear. It is believed that explaining neutrino mass will require anadditional 7 or 8 constants, which are also arbitrary parameters;

• The Higgs mechanism gives rise to the hierarchy problem if any new physics (such as quantum gravity) is presentat high energy scales. In order for the weak scale to be much smaller than the Planck scale, severe fine tuning ofStandard Model parameters is required;

• It should be modified so as to be consistent with the emerging "standard model of cosmology." In particular, theStandard Model cannot explain the observed amount of cold dark matter (CDM) and gives contributions to darkenergy which are far too large. It is also difficult to accommodate the observed predominance of matter overantimatter (matter/antimatter asymmetry). The isotropy and homogeneity of the visible universe over largedistances seems to require a mechanism like cosmic inflation, which would also constitute an extension of theStandard Model.

Currently no proposed Theory of everything has been conclusively verified.

Notes and references

Notes[1] S.L. Glashow (1961). "Partial-symmetries of weak interactions". Nuclear Physics 22: 579–588. Bibcode 1961NucPh..22..579G.

doi:10.1016/0029-5582(61)90469-2.[2] S. Weinberg (1967). "A Model of Leptons". Physical Review Letters 19: 1264–1266. Bibcode 1967PhRvL..19.1264W.

doi:10.1103/PhysRevLett.19.1264.[3] A. Salam (1968). N. Svartholm. ed. Elementary Particle Physics: Relativistic Groups and Analyticity. Eighth Nobel Symposium. Stockholm:

Almquvist and Wiksell. pp. 367.[4] F. Englert, R. Brout (1964). "Broken Symmetry and the Mass of Gauge Vector Mesons". Physical Review Letters 13: 321–323.

Bibcode 1964PhRvL..13..321E. doi:10.1103/PhysRevLett.13.321.[5] P.W. Higgs (1964). "Broken Symmetries and the Masses of Gauge Bosons". Physical Review Letters 13: 508–509.

Bibcode 1964PhRvL..13..508H. doi:10.1103/PhysRevLett.13.508.[6] G.S. Guralnik, C.R. Hagen, T.W.B. Kibble (1964). "Global Conservation Laws and Massless Particles". Physical Review Letters 13:

585–587. Bibcode 1964PhRvL..13..585G. doi:10.1103/PhysRevLett.13.585.[7] F.J. Hasert et al. (1973). "Search for elastic muon-neutrino electron scattering". Physics Letters B 46: 121. Bibcode 1973PhLB...46..121H.

doi:10.1016/0370-2693(73)90494-2.[8] F.J. Hasert et al. (1973). "Observation of neutrino-like interactions without muon or electron in the gargamelle neutrino experiment". Physics

Letters B 46: 138. Bibcode 1973PhLB...46..138H. doi:10.1016/0370-2693(73)90499-1.[9] F.J. Hasert et al. (1974). "Observation of neutrino-like interactions without muon or electron in the Gargamelle neutrino experiment". Nuclear

Physics B 73: 1. Bibcode 1974NuPhB..73....1H. doi:10.1016/0550-3213(74)90038-8.[10] D. Haidt (4 October 2004). "The discovery of the weak neutral currents" (http:/ / cerncourier. com/ cws/ article/ cern/ 29168). CERN

Courier. . Retrieved 2008-05-08.[11] "Details can be worked out if the situation is simple enough for us to make an approximation, which is almost never, but often we can

understand more or less what is happening." from The Feynman Lectures on Physics, Vol 1. pp. 2–7[12] Technically, there are nine such color–anticolor combinations. However, there is one color-symmetric combination that can be constructed

out of a linear superposition of the nine combinations, reducing the count to eight.[13] F. Englert, R. Brout (1964). "Broken Symmetry and the Mass of Gauge Vector Mesons". Physical Review Letters 13: 321–323.

Bibcode 1964PhRvL..13..321E. doi:10.1103/PhysRevLett.13.321.[14] P.W. Higgs (1964). "Broken Symmetries and the Masses of Gauge Bosons". Physical Review Letters 13: 508–509.

Bibcode 1964PhRvL..13..508H. doi:10.1103/PhysRevLett.13.508.[15] G.S. Guralnik, C.R. Hagen, T.W.B. Kibble (1964). "Global Conservation Laws and Massless Particles". Physical Review Letters 13:

585–587. Bibcode 1964PhRvL..13..585G. doi:10.1103/PhysRevLett.13.585.[16] G.S. Guralnik (2009). "The History of the Guralnik, Hagen and Kibble development of the Theory of Spontaneous Symmetry Breaking and

Gauge Particles". International Journal of Modern Physics A 24: 2601–2627. arXiv:0907.3466. Bibcode 2009IJMPA..24.2601G.doi:10.1142/S0217751X09045431.

Standard Model 18

[17] (http:/ / cms. web. cern. ch/ cms/ News/ 2011/ LP11/ )[18] (http:/ / atlas. web. cern. ch/ Atlas/ GROUPS/ PHYSICS/ CONFNOTES/ ATLAS-CONF-2011-135/ )[19] (http:/ / www. zdnet. co. uk/ news/ emerging-tech/ 2011/ 07/ 25/ cern-higgs-boson-answer-to-come-by-end-of-2012-40093510/ )[20] The normalization Q = I3 + Y is sometimes used instead.[21] http:/ / operaweb. lngs. infn. it/ spip. php?rubrique14 31May2010 Press Release.[22] "Neutrino 'ghost particle' sized up by astronomers" (http:/ / www. bbc. co. uk/ news/ 10364160). BBC News. 22 June 2010. .[23] W.-M. Yao et al. (Particle Data Group) (2006). "Review of Particle Physics: Quarks" (http:/ / pdg. lbl. gov/ 2006/ tables/ qxxx. pdf). Journal

of Physics G 33: 1. arXiv:astro-ph/0601168. Bibcode 2006JPhG...33....1Y. doi:10.1088/0954-3899/33/1/001. .[24] W.-M. Yao et al. (Particle Data Group) (2006). "Review of Particle Physics: Neutrino mass, mixing, and flavor change" (http:/ / pdg. lbl.

gov/ 2007/ reviews/ numixrpp. pdf). Journal of Physics G 33: 1. arXiv:astro-ph/0601168. Bibcode 2006JPhG...33....1Y.doi:10.1088/0954-3899/33/1/001. .

[25] CERN Press Release (http:/ / press. web. cern. ch/ press/ PressReleases/ Releases2010/ PR08. 10E. html)

References

Further reading• R. Oerter (2006). The Theory of Almost Everything: The Standard Model, the Unsung Triumph of Modern

Physics. Plume.• B.A. Schumm (2004). Deep Down Things: The Breathtaking Beauty of Particle Physics. Johns Hopkins

University Press. ISBN 0-8018-7971-X.Introductory textbooks• I. Aitchison, A. Hey (2003). Gauge Theories in Particle Physics: A Practical Introduction.. Institute of Physics.

ISBN 9780585445502.• W. Greiner, B. Müller (2000). Gauge Theory of Weak Interactions. Springer. ISBN 3-540-67672-4.• G.D. Coughlan, J.E. Dodd, B.M. Gripaios (2006). The Ideas of Particle Physics: An Introduction for Scientists.

Cambridge University Press.• D.J. Griffiths (1987). Introduction to Elementary Particles. John Wiley & Sons. ISBN 0-471-60386-4.• G.L. Kane (1987). Modern Elementary Particle Physics. Perseus Books. ISBN 0-201-11749-5.Advanced textbooks• T.P. Cheng, L.F. Li (2006). Gauge theory of elementary particle physics. Oxford University Press.

ISBN 0-19-851961-3. Highlights the gauge theory aspects of the Standard Model.• J.F. Donoghue, E. Golowich, B.R. Holstein (1994). Dynamics of the Standard Model. Cambridge University

Press. ISBN 978-0521476522. Highlights dynamical and phenomenological aspects of the Standard Model.• L. O'Raifeartaigh (1988). Group structure of gauge theories. Cambridge University Press. ISBN 0-521-34785-8.

Highlights group-theoretical aspects of the Standard Model.Journal articles• E.S. Abers, B.W. Lee (1973). "Gauge theories". Physics Reports 9: 1–141. Bibcode 1973PhR.....9....1A.

doi:10.1016/0370-1573(73)90027-6.• Y. Hayato et al. (1999). "Search for Proton Decay through p → νK+ in a Large Water Cherenkov Detector".

Physical Review Letters 83: 1529. arXiv:hep-ex/9904020. Bibcode 1999PhRvL..83.1529H.doi:10.1103/PhysRevLett.83.1529.

• S.F. Novaes (2000). "Standard Model: An Introduction". arXiv:hep-ph/0001283 [hep-ph].• D.P. Roy (1999). "Basic Constituents of Matter and their Interactions — A Progress Report.".

arXiv:hep-ph/9912523 [hep-ph].• F. Wilczek (2004). "The Universe Is A Strange Place". arXiv:astro-ph/0401347 [astro-ph].

Standard Model 19

External links• " Standard Model may be found incomplete, (http:/ / www. newscientist. com/ news/ news. jsp?id=ns9999404)"

New Scientist.• " Observation of the Top Quark (http:/ / www-cdf. fnal. gov/ top_status/ top. html)" at Fermilab.• " The Standard Model Lagrangian. (http:/ / cosmicvariance. com/ 2006/ 11/ 23/ thanksgiving)" After electroweak

symmetry breaking, with no explicit Higgs boson.• " Standard Model Lagrangian (http:/ / nuclear. ucdavis. edu/ ~tgutierr/ files/ stmL1. html)" with explicit Higgs

terms. PDF, PostScript, and LaTeX versions.• " The particle adventure. (http:/ / particleadventure. org/ )" Web tutorial.• Nobes, Matthew (2002) "Introduction to the Standard Model of Particle Physics" on Kuro5hin: Part 1, (http:/ /

www. kuro5hin. org/ story/ 2002/ 5/ 1/ 3712/ 31700) Part 2, (http:/ / www. kuro5hin. org/ story/ 2002/ 5/ 14/19363/ 8142) Part 3a, (http:/ / www. kuro5hin. org/ story/ 2002/ 7/ 15/ 173318/ 784) Part 3b. (http:/ / www.kuro5hin. org/ story/ 2002/ 8/ 21/ 195035/ 576)

SupersymmetryIn particle physics, supersymmetry (often abbreviated SUSY) is a symmetry that relates elementary particles of onespin to other particles that differ by half a unit of spin and are known as superpartners. In a theory with unbrokensupersymmetry, for every type of boson there exists a corresponding type of fermion with the same mass and internalquantum numbers, and vice-versa.There is no direct evidence for the existence of supersymmetry.[1] It is motivated by possible solutions to severaltheoretical problems. Since the superpartners of the Standard Model particles have not been observed,supersymmetry, if it exists, must be a broken symmetry, allowing the superparticles to be heavier than thecorresponding Standard Model particles.If supersymmetry exists close to the TeV energy scale, it allows for a solution of the hierarchy problem of theStandard Model, i.e., the fact that the Higgs boson mass is subject to quantum corrections which — barringextremely fine-tuned cancellations among independent contributions — would make it so large as to undermine theinternal consistency of the theory. In supersymmetric theories, on the other hand, the contributions to the quantumcorrections coming from Standard Model particles are naturally canceled by the contributions of the correspondingsuperpartners. Other attractive features of TeV-scale supersymmetry are the fact that it allows for the high-energyunification of the weak interactions, the strong interactions and electromagnetism, and the fact that it provides acandidate for Dark Matter and a natural mechanism for electroweak symmetry breaking. Therefore, scenarios wheresupersymmetric partners appear with masses not much greater than 1 TeV are considered the most well-motivated bytheorists[2] . These scenarios would imply that experimental traces of the superpartners should begin to emerge inhigh-energy collisions at the LHC relatively soon. As of September 2011, no meaningful signs of the superpartnershave been observed[3] [4] , which is beginning to significantly constrain the most popular incarnations ofsupersymmetry. However, the total parameter space of consistent supersymmetric extensions of the Standard Modelis extremely diverse and can not be definitively ruled out at the LHC.Another theoretically appealing property of supersymmetry is that it offers the only "loophole" to theColeman–Mandula theorem, which prohibits spacetime and internal symmetries from being combined in anynontrivial way, for quantum field theories like the Standard Model under very general assumptions. TheHaag-Lopuszanski-Sohnius theorem demonstrates that supersymmetry is the only way spacetime and internalsymmetries can be consistently combined.[5]

In general, supersymmetric quantum field theory is often much easier to work with, as many more problems become exactly solvable. Supersymmetry is also a feature of most versions of string theory, though it may exist in nature

Supersymmetry 20

even if string theory is incorrect.The Minimal Supersymmetric Standard Model is one of the best studied candidates for physics beyond the StandardModel. Theories of gravity that are also invariant under supersymmetry are known as supergravity theories.

HistoryA supersymmetry relating mesons and baryons was first proposed, in the context of hadronic physics, by HironariMiyazawa in 1966, but his work was ignored at the time.[6] [7] [8] [9] In the early 1970s, J. L. Gervais and B. Sakita(in 1971), Yu. A. Golfand and E.P. Likhtman (also in 1971), D.V. Volkov and V.P. Akulov (in 1972) and J. Wessand B. Zumino (in 1974) independently rediscovered supersymmetry, a radically new type of symmetry of spacetimeand fundamental fields, which establishes a relationship between elementary particles of different quantum nature,bosons and fermions, and unifies spacetime and internal symmetries of the microscopic world. Supersymmetry firstarose in 1971 in the context of an early version of string theory by Pierre Ramond, John H. Schwarz and AndreNeveu, but the mathematical structure of supersymmetry has subsequently been applied successfully to other areasof physics; firstly by Wess, Zumino, and Abdus Salam and their fellow researchers to particle physics, and later to avariety of fields, ranging from quantum mechanics to statistical physics. It remains a vital part of many proposedtheories of physics.The first realistic supersymmetric version of the Standard Model was proposed in 1981 by Howard Georgi and SavasDimopoulos and is called the Minimal Supersymmetric Standard Model or MSSM for short. It was proposed to solvethe hierarchy problem and predicts superpartners with masses between 100 GeV and 1 TeV. As of 2009 there is noirrefutable experimental evidence that supersymmetry is a symmetry of nature. Since 2010, the Large HadronCollider at CERN is producing the world's highest energy collisions and offers the best chance at discoveringsuperparticles for the foreseeable future.

Applications

Extension of possible symmetry groupsOne reason that physicists explored supersymmetry is because it offers an extension to the more familiar symmetriesof quantum field theory. These symmetries are grouped into the Poincaré group and internal symmetries and theColeman–Mandula theorem showed that under certain assumptions, the symmetries of the S-matrix must be a directproduct of the Poincaré group with a compact internal symmetry group or if there is no mass gap, the conformalgroup with a compact internal symmetry group. In 1971 Golfand and Likhtman were the first to show that thePoincaré algebra can be extended through introduction of four anticommuting spinor generators (in fourdimensions), which later became known as supercharges. In 1975 the Haag-Lopuszanski-Sohnius theorem analyzedall possible superalgebras in the general form, including those with an extended number of the supergenerators andcentral charges. This extended super-Poincaré algebra paved the way for obtaining a very large and important classof supersymmetric field theories.

The supersymmetry algebra

Traditional symmetries in physics are generated by objects that transform under the tensor representations of thePoincaré group and internal symmetries. Supersymmetries, on the other hand, are generated by objects that transformunder the spinor representations. According to the spin-statistics theorem, bosonic fields commute while fermionicfields anticommute. Combining the two kinds of fields into a single algebra requires the introduction of a Z2-gradingunder which the bosons are the even elements and the fermions are the odd elements. Such an algebra is called a Liesuperalgebra.The simplest supersymmetric extension of the Poincaré algebra is the Super-Poincaré algebra. Expressed in terms oftwo Weyl spinors, has the following anti-commutation relation:

Supersymmetry 21

and all other anti-commutation relations between the Qs and commutation relations between the Qs and Ps vanish. Inthe above expression are the generators of translation and are the Pauli matrices.There are representations of a Lie superalgebra that are analogous to representations of a Lie algebra. Each Liealgebra has an associated Lie group and a Lie superalgebra can sometimes be extended into representations of a Liesupergroup.

The Supersymmetric Standard ModelIncorporating supersymmetry into the Standard Model requires doubling the number of particles since there is noway that any of the particles in the Standard Model can be superpartners of each other. With the addition of newparticles, there are many possible new interactions. The simplest possible supersymmetric model consistent with theStandard Model is the Minimal Supersymmetric Standard Model (MSSM) which can include the necessaryadditional new particles that are able to be superpartners of those in the Standard Model.

Cancellation of the Higgs boson quadratic mass renormalization between fermionictop quark loop and scalar stop squark tadpole Feynman diagrams in a

supersymmetric extension of the Standard Model

One of the main motivations for SUSYcomes from the quadratically divergentcontributions to the Higgs mass squared.The quantum mechanical interactions of theHiggs boson causes a large renormalizationof the Higgs mass and unless there is anaccidental cancellation, the natural size ofthe Higgs mass is the highest scale possible.This problem is known as the hierarchyproblem. Supersymmetry reduces the size ofthe quantum corrections by havingautomatic cancellations between fermionicand bosonic Higgs interactions. Ifsupersymmetry is restored at the weak scale,then the Higgs mass is related tosupersymmetry breaking which can beinduced from small non-perturbative effectsexplaining the vastly different scales in theweak interactions and gravitational interactions.

In many supersymmetric Standard Models there is a heavy stable particle (such as neutralino) which could serve as aWeakly interacting massive particle (WIMP) dark matter candidate. The existence of a supersymmetric dark mattercandidate is closely tied to R-parity.

The standard paradigm for incorporating supersymmetry into a realistic theory is to have the underlying dynamics ofthe theory be supersymmetric, but the ground state of the theory does not respect the symmetry and supersymmetryis broken spontaneously. The supersymmetry break can not be done permanently by the particles of the MSSM asthey currently appear. This means that there is a new sector of the theory that is responsible for the breaking. Theonly constraint on this new sector is that it must break supersymmetry permanently and must give superparticles TeVscale masses. There are many models that can do this and most of their details do not currently matter. In order toparameterize the relevant features of supersymmetry breaking, arbitrary soft SUSY breaking terms are added to thetheory which temporarily break SUSY explicitly but could never arise from a complete theory of supersymmetrybreaking.

Supersymmetry 22

Gauge Coupling Unification

One piece of evidence for supersymmetry existing is gauge coupling unification. The renormalization groupevolution of the three gauge coupling constants of the Standard Model is somewhat sensitive to the present particlecontent of the theory. These coupling constants do not quite meet together at a common energy scale if we run therenormalization group using the Standard Model.[1] With the addition of minimal SUSY joint convergence of thecoupling constants is projected at approximately 1016 GeV.[1]

Supersymmetric quantum mechanicsSupersymmetric quantum mechanics adds the SUSY superalgebra to quantum mechanics as opposed to quantumfield theory. Supersymmetric quantum mechanics often comes up when studying the dynamics of supersymmetricsolitons and due to the simplified nature of having fields only functions of time (rather than space-time), a great dealof progress has been made in this subject and is now studied in its own right.SUSY quantum mechanics involves pairs of Hamiltonians which share a particular mathematical relationship, whichare called partner Hamiltonians. (The potential energy terms which occur in the Hamiltonians are then calledpartner potentials.) An introductory theorem shows that for every eigenstate of one Hamiltonian, its partnerHamiltonian has a corresponding eigenstate with the same energy. This fact can be exploited to deduce manyproperties of the eigenstate spectrum. It is analogous to the original description of SUSY, which referred to bosonsand fermions. We can imagine a "bosonic Hamiltonian", whose eigenstates are the various bosons of our theory. TheSUSY partner of this Hamiltonian would be "fermionic", and its eigenstates would be the theory's fermions. Eachboson would have a fermionic partner of equal energy.SUSY concepts have provided useful extensions to the WKB approximation. In addition, SUSY has been applied tonon-quantum statistical mechanics through the Fokker-Planck equation.

MathematicsSUSY is also sometimes studied mathematically for its intrinsic properties. This is because it describes complexfields satisfying a property known as holomorphy, which allows holomorphic quantities to be exactly computed.This makes supersymmetric models useful toy models of more realistic theories. A prime example of this has beenthe demonstration of S-duality in four-dimensional gauge theories that interchanges particles and monopoles.

General supersymmetrySupersymmetry appears in many different contexts in theoretical physics that are closely related. It is possible tohave multiple supersymmetries and also have supersymmetric extra dimensions.

Extended supersymmetryIt is possible to have more than one kind of supersymmetry transformation. Theories with more than onesupersymmetry transformation are known as extended supersymmetric theories. The more supersymmetry a theoryhas, the more constrained the field content and interactions are. Typically the number of copies of a supersymmetryis a power of 2, i.e. 1, 2, 4, 8. In four dimensions, a spinor has four degrees of freedom and thus the minimal numberof supersymmetry generators is four in four dimensions and having eight copies of supersymmetry means that thereare 32 supersymmetry generators.The maximal number of supersymmetry generators possible is 32. Theories with more than 32 supersymmetrygenerators automatically have massless fields with spin greater than 2. It is not known how to make massless fieldswith spin greater than two interact, so the maximal number of supersymmetry generators considered is 32. Thiscorresponds to an N = 8 supersymmetry theory. Theories with 32 supersymmetries automatically have a graviton.

Supersymmetry 23

In four dimensions there are the following theories, with the corresponding multiplets[10] (CPT adds a copy,whenever they are not invariant under such symmetry)• N = 1Chiral multiplet: (0,1⁄2) Vector multiplet: (1⁄2,1) Gravitino multiplet: (1,3⁄2) Graviton multiplet: (3⁄2,2)• N = 2hypermultiplet: (-1⁄2,02,1⁄2) vector multiplet: (0,1⁄2

2,1) supergravity multiplet: (1,3⁄22,2)

• N = 4Vector multiplet: (-1,-1⁄2

4,06,1⁄24,1) Supergravity multiplet: (0,1⁄2

4,16,3⁄24,2)

• N = 8Supergravity multiplet: (-2,-3⁄2

8,-128,-1⁄256,070,1⁄2

56,128,3⁄28,2)

Supersymmetry in alternate numbers of dimensionsIt is possible to have supersymmetry in dimensions other than four. Because the properties of spinors changedrastically between different dimensions, each dimension has its characteristic. In d dimensions, the size of spinors isroughly 2d/2 or 2(d − 1)/2. Since the maximum number of supersymmetries is 32, the greatest number of dimensions inwhich a supersymmetric theory can exist is eleven.

Supersymmetry as a quantum groupSupersymmetry can be reinterpreted in the language of noncommutative geometry and quantum groups. Inparticular, it involves a mild form of noncommutativity, namely supercommutativity. See the main article for moredetails.

Supersymmetry in quantum gravitySupersymmetry is part of a larger enterprise of theoretical physics to unify everything we know about the physicalworld into a single fundamental framework of physical laws, known as the quest for a Theory of Everything (TOE).A significant part of this larger enterprise is the quest for a theory of quantum gravity, which would unify theclassical theory of general relativity and the Standard Model, which explains the other three basic forces in physics(electromagnetism, the strong interaction, and the weak interaction), and provides a palette of fundamental particlesupon which all four forces act. Two of the most active approaches to forming a theory of quantum gravity are stringtheory and loop quantum gravity (LQG), although in theory, supersymmetry could be a component of othertheoretical approaches as well.For string theory to be consistent, supersymmetry appears to be required at some level (although it may be a stronglybroken symmetry). In particle theory, supersymmetry is recognized as a way to stabilize the hierarchy between theunification scale and the electroweak scale (or the Higgs boson mass), and can also provide a natural dark mattercandidate. String theory also requires extra spatial dimensions which have to be compactified as in Kaluza-Kleintheory.Loop quantum gravity (LQG), in its current formulation, predicts no additional spatial dimensions, nor anything elseabout particle physics. These theories can be formulated in three spatial dimensions and one dimension of time,although in some LQG theories dimensionality is an emergent property of the theory, rather than a fundamentalassumption of the theory. Also, LQG is a theory of quantum gravity which does not require supersymmetry. LeeSmolin, one of the originators of LQG, has proposed that a loop quantum gravity theory incorporating eithersupersymmetry or extra dimensions, or both, be called "loop quantum gravity II".If experimental evidence confirms supersymmetry in the form of supersymmetric particles such as the neutralino that is often believed to be the lightest superpartner, some people believe this would be a major boost to string theory.

Supersymmetry 24

Since supersymmetry is a required component of string theory, any discovered supersymmetry would be consistentwith string theory. If the Large Hadron Collider and other major particle physics experiments fail to detectsupersymmetric partners or evidence of extra dimensions, many versions of string theory which had predicted certainlow mass superpartners to existing particles may need to be significantly revised. The failure of experiments todiscover either supersymmetric partners or extra spatial dimensions, as of 2009, has encouraged loop quantumgravity researchers.

Current LimitsThe tightest limits will of course come from direct production at colliders. Both the Large Electron–Positron Colliderand Tevatron had set limits for specific models which have now been exceeded by the Large Hadron Collider.Searches are only applicable for a finite set of tested points because simulation using the Monte Carlo method mustbe made so that limits for that particular model can be calculated. This complicates matters because differentexperiments have looked at different sets of points. Some extrapolation between points can be made within particularmodels but it is difficult to set general limits even for the Minimal Supersymmetric Standard Model.The first mass limits for squarks and gluinos were made at CERN by the UA1 experiment and the UA2 experimentat the Super Proton Synchrotron. LEP later set very strong limits.[11] In 2006 these limits were extended by the D0experiment[12] [13] The LHC has now extended these limits, by extending the search area, with no sign ofsupersymmmetry.[14] [15] [16] [17]

References[1] Gordon L. Kane, The Dawn of Physics Beyond the Standard Model, Scientific American, June 2003, page 60 and The frontiers of physics,

special edition, Vol 15, #3, page 8 "Indirect evidence for supersymmetry comes from the extrapolation of interactions to high energies."[2] (http:/ / profmattstrassler. com/ articles-and-posts/ lhcposts/ what-do-current-mid-august-2011-lhc-results-imply-about-supersymmetry/ )[3] ATLAS SUSY search documents (https:/ / twiki. cern. ch/ twiki/ bin/ view/ AtlasPublic/

SupersymmetryPublicResults#Early_2011_Data_5_CONF_Notes)[4] CMS SUSY search documents (https:/ / twiki. cern. ch/ twiki/ bin/ view/ CMSPublic/ PhysicsResultsSUS)[5] R. Haag, J. T. Lopuszanski and M. Sohnius, " All Possible Generators Of Supersymmetries Of The S Matrix (http:/ / www. sciencedirect.

com/ science/ article/ B6TVC-4718W97-YF/ 1/ bc160d55fb6a0faddac181fcff6871ce)", Nucl. Phys. B 88 (1975) 257[6] H. Miyazawa (1966). "Baryon Number Changing Currents". Prog. Theor. Phys. 36 (6): 1266–1276. Bibcode 1966PThPh..36.1266M.

doi:10.1143/PTP.36.1266.[7] H. Miyazawa (1968). "Spinor Currents and Symmetries of Baryons and Mesons". Phys. Rev. 170 (5): 1586–1590.

Bibcode 1968PhRv..170.1586M. doi:10.1103/PhysRev.170.1586.[8] Michio Kaku, Quantum Field Theory, ISBN 0-19-509158-2, pg 663.[9] Peter Freund, Introduction to Supersymmetry, ISBN 0-521-35675-X, pages 26-27, 138.[10] Polchinski,J. String theory. Vol. 2: Superstring theory and beyond, Appendix B[11] LEPSUSYWG, ALEPH, DELPHI, L3 and OPAL experiments, Charginos, large m0 LEPSUSYWG/01-03.1[12] The D0-Collaboration, Search for associated production of charginos and neutralinos in the trilepton final state using 2.3 $fb^-1$ of data,

arXiv:/0901.0646 [hep-ex][13] The D0 Collaboration, V. Abazov, et al., Search for Squarks and Gluinos in events with jets and missing transverse energy using 2.1 $fb^-1$

of $p\bar{p}$ collision data at $\sqrt{s}$ = 1.96 TeV , arXiv:0712.3805v2 [hep-ex][14] Implications of Initial LHC Searches for Supersymmetry (http:/ / www. math. columbia. edu/ ~woit/ wordpress/ ?p=3479)[15] Fine-tuning implications for complementary dark matter and LHC SUSY searches (http:/ / arxiv. org/ abs/ 1101. 4664)[16] What LHC tells about SUSY (http:/ / resonaances. blogspot. com/ 2011/ 02/ what-lhc-tells-about-susy. html)[17] Early SUSY searches at the LHC (http:/ / www. hep. ph. ic. ac. uk/ susytalks/ iop-susytapper. pdf)

Supersymmetry 25

Further reading• A Supersymmetry Primer (http:/ / arxiv. org/ abs/ hep-ph/ 9709356) by S. Martin, 2011• Introduction to Supersymmetry (http:/ / arxiv. org/ pdf/ hep-th/ 9612114) By Joseph D. Lykken, 1996• An Introduction to Supersymmetry (http:/ / arxiv. org/ pdf/ hep-ph/ 9611409) By Manuel Drees, 1996• Introduction to Supersymmetry (http:/ / arxiv. org/ pdf/ hep-th/ 0101055) By Adel Bilal, 2001• An Introduction to Global Supersymmetry (http:/ / www. physics. uc. edu/ ~argyres/ 661/ susy2001. pdf) by

Philip Arygres, 2001• Weak Scale Supersymmetry (http:/ / www. cambridge. org/ uk/ catalogue/ catalogue. asp?isbn=0521857864) by

Howard Baer and Xerxes Tata, 2006.• Cooper, F., A. Khare and U. Sukhatme. "Supersymmetry in Quantum Mechanics." Phys. Rep. 251 (1995) 267-85

(arXiv:hep-th/9405029).• Junker, G. Supersymmetric Methods in Quantum and Statistical Physics, Springer-Verlag (1996).• Gordon L. Kane.Supersymmetry: Unveiling the Ultimate Laws of Nature Basic Books, New York (2001). ISBN

0-7382-0489-7.• Gordon L. Kane and Shifman, M., eds. The Supersymmetric World: The Beginnings of the Theory, World

Scientific, Singapore (2000). ISBN 981-02-4522-X.• D.V. Volkov, V.P. Akulov, Pisma Zh.Eksp.Teor.Fiz. 16 (1972) 621; Phys.Lett. B46 (1973) 109.• V.P. Akulov, D.V. Volkov, Teor.Mat.Fiz. 18 (1974) 39.• Weinberg, Steven, The Quantum Theory of Fields, Volume 3: Supersymmetry, Cambridge University Press,

Cambridge, (1999). ISBN 0-521-66000-9.• Wess, Julius, and Jonathan Bagger, Supersymmetry and Supergravity, Princeton University Press, Princeton,

(1992). ISBN 0-691-02530-4.• Bennett GW, et al.; Muon (g−2) Collaboration (2004). "Measurement of the negative muon anomalous magnetic

moment to 0.7 ppm". Physical Review Letters 92 (16): 161802. arXiv:hep-ex/0401008.Bibcode 2004PhRvL..92p1802B. doi:10.1103/PhysRevLett.92.161802. PMID 15169217.

• Brookhaven National Laboratory (Jan. 8, 2004). New g−2 measurement deviates further from Standard Model(http:/ / www. bnl. gov/ bnlweb/ pubaf/ pr/ 2004/ bnlpr010804. htm). Press Release.

• Fermi National Accelerator Laboratory (Sept 25, 2006). Fermilab's CDF scientists have discovered thequick-change behavior of the B-sub-s meson. (http:/ / www. fnal. gov/ pub/ presspass/ press_releases/CDF_meson. html) Press Release.

External links• What do current LHC results (mid-August 2011) imply about supersymmetry? (http:/ / profmattstrassler. com/

articles-and-posts/ lhcposts/ what-do-current-mid-august-2011-lhc-results-imply-about-supersymmetry/ ) MattStrassler

• ATLAS Experiment Supersymmetry search documents (https:/ / twiki. cern. ch/ twiki/ bin/ view/ AtlasPublic/SupersymmetryPublicResults#Early_2011_Data_5_CONF_Notes)

• CMS Experiment Supersymmetry search documents (https:/ / twiki. cern. ch/ twiki/ bin/ view/ CMSPublic/PhysicsResultsSUS)

• "Particle wobble shakes up supersymmetry" (http:/ / www. cosmosmagazine. com/ node/ 714), Cosmos magazine,September 2006

• LHC results put supersymmetry theory 'on the spot' (http:/ / www. bbc. co. uk/ news/science-environment-14680570) BBC news 27/8/2011

Elementary particle 26

Elementary particle

Standard Model of Elementary Particles

In particle physics, an elementary particleor fundamental particle is a particle notknown to have substructure; that is, it is notknown to be made up of smaller particles. Ifan elementary particle truly has nosubstructure, then it is one of the basicbuilding blocks of the universe from whichall other particles are made. In the StandardModel, the elementary particles consist ofthe fundamental fermions (including quarks,leptons, and their antiparticles), and thefundamental bosons (including gaugebosons and the Higgs boson).[1] [2]

Historically, the hadrons (mesons andbaryons such as the proton and neutron) andeven whole atoms were once regarded aselementary particles. A central feature inelementary particle theory is the early 20thcentury idea of "quanta", whichrevolutionized the understanding ofelectromagnetic radiation and brought aboutquantum mechanics. For mathematical purposes, elementary particles are normally treated as point particles,although some particle theories such as string theory posit a physical dimension.

Overview

An overview of the various families of elementary and composite particles, and thetheories describing their interactions

According to the Standard Model, allelementary particles are either bosonsor fermions (depending on their spin).The spin-statistics theorem identifiesthe resulting quantum statistics thatdifferentiates fermions from bosons.According to this methodology:Particles normally associated withmatter are fermions. They havehalf-integer spin and are divided intotwelve flavours. Particles associatedwith fundamental forces are bosonsand they have integer spin.[3]

Elementary fermions (matterparticles)

• Quarks:• up, down, charm, strange, top, bottom

Elementary particle 27

• Leptons:• electron, electron neutrino, muon, muon neutrino, tau, tau neutrino

Elementary bosons (force-carrying particles)

• Gauge bosons:• gluon, W and Z bosons, photon

Other bosons

• Higgs bosonOf these, only the Higgs boson remains undiscovered, but efforts are being taken at the Large Hadron Collider todetermine whether it exists or not. Additional elementary particles may exist, such as the graviton, which wouldmediate gravitation. Such particles lie beyond the Standard Model.

Common elementary particlesSeveral estimates imply that practically all the matter, when measured by mass, in the visible universe (not includingdark matter) is in the protons of hydrogen atoms, and that roughly 1080 protons exist in the visible universe(Eddington number), and roughly 1080 atoms exist in the visible universe.[4] Each proton is, in turn, composed of 3elementary particles: two up quarks and one down quark. Neutrons and other particles heavier than protons, as wellas helium and other atoms with more than one proton, are so rare that their total mass in the visible universe is muchless than the total mass of protons in hydrogen atoms. Lighter particles of matter, although equal (electrons) or vastlymore (neutrinos) numerous than protons, are so much lighter than protons, that their total mass in the visible universeis again much less than the total mass of all protons.Some estimates imply that practically all the matter, when measured by numbers of particles, in the visible universe(not including dark matter) is in the form of neutrinos, and that roughly 1086 elementary particles of matter exist inthe visible universe, mostly neutrinos.[5] Some estimates imply that roughly 1097 elementary particles exist in thevisible universe (not including dark matter), mostly photons, gravitons, and other massless force carriers.[5]

Standard ModelThe Standard Model of particle physics contains 12 flavors of elementary fermions, plus their correspondingantiparticles, as well as elementary bosons that mediate the forces and the still undiscovered Higgs boson. However,the Standard Model is widely considered to be a provisional theory rather than a truly fundamental one, since it is notknown if it is compatible with Einstein's general relativity. There are likely to be hypothetical elementary particlesnot described by the Standard Model, such as the graviton, the particle that would carry the gravitational force or thesparticles, supersymmetric partners of the ordinary particles.

Fundamental fermionsThe 12 fundamental fermionic flavours are divided into three generations of four particles each. Six of the particlesare quarks. The remaining six are leptons, three of which are neutrinos, and the remaining three of which have anelectric charge of −1: the electron and its two cousins, the muon and the tau.

Elementary particle 28

Particle Generations

Leptons

First generation Second generation Third generation

Name Symbol Name Symbol Name Symbol

electron e− muon μ− tau τ−

electron neutrino νe muon neutrino νμ tau neutrino ντ

Quarks

First generation Second generation Third generation

up quark u charm quark c top quark t

down quark d strange quark s bottom quark b

Antiparticles

There are also 12 fundamental fermionic antiparticles that correspond to these 12 particles. The antielectron(positron) e+ is the electron's antiparticle and has an electric charge of +1 and so on:

Particle Generations

Antileptons

First generation Second generation Third generation

Name Symbol Name Symbol Name Symbol

antielectron (positron) e+ antimuon μ+ antitau τ+

electron antineutrino νe muon antineutrino νμ tau antineutrino ντ

Antiquarks

First generation Second generation Third generation

up antiquark u charm antiquark c top antiquark t

down antiquark d strange antiquark s bottom antiquark b

Quarks

Isolated quarks and antiquarks have never been detected, a fact explained by confinement. Every quark carries one ofthree color-charges of the strong interaction; antiquarks similarly carry anticolor. Color-charged particles interact viagluon exchange in the same way that charged particles interact via photon exchange. However, gluons arethemselves color-charged, resulting in an amplification of the strong force as color-charged particles are separated.Unlike the electromagnetic force, which diminishes as charged particles separate, color-charged particles feelincreasing force.However, color-charged particles may combine to form color neutral composite particles called hadrons. A quarkmay pair up with an antiquark: the quark has a color and the antiquark has the corresponding anticolor. The color andanticolor cancel out, forming a color neutral meson. Alternatively, three quarks can exist together, one quark being"red", another "blue", another "green". These three colored quarks together form a color-neutral baryon.Symmetrically, three antiquarks with the colors "antired", "antiblue" and "antigreen" can form a color-neutralantibaryon.Quarks also carry fractional electric charges, but, since they are confined within hadrons whose charges are all integral, fractional charges have never been isolated. Note that quarks have electric charges of either +2/3 or −1/3,

Elementary particle 29

whereas antiquarks have corresponding electric charges of either −2/3 or +1/3.Evidence for the existence of quarks comes from deep inelastic scattering: firing electrons at nuclei to determine thedistribution of charge within nucleons (which are baryons). If the charge is uniform, the electric field around theproton should be uniform and the electron should scatter elastically. Low-energy electrons do scatter in this way, but,above a particular energy, the protons deflect some electrons through large angles. The recoiling electron has muchless energy and a jet of particles is emitted. This inelastic scattering suggests that the charge in the proton is notuniform but split among smaller charged particles: quarks.

Fundamental bosonsIn the Standard Model, vector (spin-1) bosons (gluons, photons, and the W and Z bosons) mediate forces, whereasthe Higgs boson (spin-0) is responsible for the intrinsic mass of particles.

Gluons

Gluons are the mediators of the strong interaction and carry both colour and anticolour. Although gluons aremassless, they are never observed in detectors due to colour confinement; rather, they produce jets of hadrons,similar to single quarks. The first evidence for gluons came from annihilations of electrons and antielectrons at highenergies, which sometimes produced three jets — a quark, an antiquark, and a gluon.

Electroweak bosons

There are three weak gauge bosons: W+, W−, and Z0; these mediate the weak interaction. The massless photonmediates the electromagnetic interaction.

Higgs boson

Although the weak and electromagnetic forces appear quite different to us at everyday energies, the two forces aretheorized to unify as a single electroweak force at high energies. This prediction was clearly confirmed bymeasurements of cross-sections for high-energy electron-proton scattering at the HERA collider at DESY. Thedifferences at low energies is a consequence of the high masses of the W and Z bosons, which in turn are aconsequence of the Higgs mechanism. Through the process of spontaneous symmetry breaking, the Higgs selects aspecial direction in electroweak space that causes three electroweak particles to become very heavy (the weakbosons) and one to remain massless (the photon). Although the Higgs mechanism has become an accepted part of theStandard Model, the Higgs boson itself has not yet been observed in detectors. Indirect evidence for the Higgs bosonsuggests its mass lies below 200-250 GeV.[6] In this case, the LHC experiments may be able to discover this lastmissing piece of the Standard Model.

Beyond the Standard ModelAlthough all experimental evidence confirms the predictions of the Standard Model, many physicists find this modelto be unsatisfactory due to its many undetermined parameters, many fundamental particles, the non-observation ofthe Higgs boson and other more theoretical considerations such as the hierarchy problem. There are manyspeculative theories beyond the Standard Model that attempt to rectify these deficiencies.

Grand unificationOne extension of the Standard Model attempts to combine the electroweak interaction with the strong interactioninto a single 'grand unified theory' (GUT). Such a force would be spontaneously broken into the three forces by aHiggs-like mechanism. The most dramatic prediction of grand unification is the existence of X and Y bosons, whichcause proton decay. However, the non-observation of proton decay at Super-Kamiokande rules out the simplestGUTs, including SU(5) and SO(10).

Elementary particle 30

SupersymmetrySupersymmetry extends the Standard Model by adding an additional class of symmetries to the Lagrangian. Thesesymmetries exchange fermionic particles with bosonic ones. Such a symmetry predicts the existence ofsupersymmetric particles, abbreviated as sparticles, which include the sleptons, squarks, neutralinos, and charginos.Each particle in the Standard Model would have a superpartner whose spin differs by 1/2 from the ordinary particle.Due to the breaking of supersymmetry, the sparticles are much heavier than their ordinary counterparts; they are soheavy that existing particle colliders would not be powerful enough to produce them. However, some physicistsbelieve that sparticles will be detected when the Large Hadron Collider at CERN begins running.

String theoryString Theory is a model of physics where all "particles" that make up matter are composed of strings (measuring atthe Planck length) that exist in an 11-dimensional (according to M-theory, the leading version) universe. Thesestrings vibrate at different frequencies that determine mass, electric charge, color charge, and spin. A string can beopen (a line) or closed in a loop (a one-dimensional sphere, like a circle). As a string moves through space it sweepsout something called a world sheet. String theory predicts 1- to 10-branes (a 1-brane being a string and a 10-branebeing a 10-dimensional object) that prevent tears in the "fabric" of space using the uncertainty principle (E.g., theelectron orbiting a hydrogen atom has the probability, albeit small, that it could be anywhere else in the universe atany given moment).String theory proposes that our universe is merely a 4-brane, inside which exist the 3 space dimensions and the 1time dimension that we observe. The remaining 6 theoretical dimensions either are very tiny and curled up (and toosmall to affect our universe in any way) or simply do not/cannot exist in our universe (because they exist in a granderscheme called the "multiverse" outside our known universe).Some predictions of the string theory include existence of extremely massive counterparts of ordinary particles dueto vibrational excitations of the fundamental string and existence of a massless spin-2 particle behaving like thegraviton.

TechnicolorTechnicolor theories try to modify the Standard model in a minimal way by introducing a new QCD-like interaction.This means one adds a new theory of so called Techniquarks, interacting via so called Technigluons. The main ideais that the Higgs-Boson is not an elementary particle but a bound state of these objects.

Preon theoryAccording to preon theory there are one or more orders of particles more fundamental than those (or most of those)found in the Standard Model. The most fundamental of these are normally called preons, which is derived from"pre-quarks". In essence, preon theory tries to do for the Standard Model what the Standard Model did for theparticle zoo that came before it. Most models assume that almost everything in the Standard Model can be explainedin terms of three to half a dozen more fundamental particles and the rules that govern their interactions. Interest inpreons has waned since the simplest models were experimentally ruled out in the 1980s.

Elementary particle 31

Acceleron theoryAccelerons are the hypothetical subatomic particles that integrally link the newfound mass of the neutrino and to thedark energy conjectured to be accelerating the expansion of the universe.[7]

In theory, neutrinos are influenced by a new force resulting from their interactions with accelerons. Dark energyresults as the universe tries to pull neutrinos apart.[7]

Notes[1] Gribbin, John (2000). Q is for Quantum - An Encyclopedia of Particle Physics. Simon & Schuster. ISBN 0-684-85578-X.[2] Clark, John, E.O. (2004). The Essential Dictionary of Science. Barnes & Noble. ISBN 0-7607-4616-8.[3] Veltman, Martinus (2003). Facts and Mysteries in Elementary Particle Physics. World Scientific. ISBN 981-238-149-X.[4] Penrose, Roger (1989). The Emperor's New Mind.[5] Munafo, Robert. "Notable Properties of Specific Numbers" (http:/ / mrob. com/ pub/ math/ numbers-19. html) by Robert Munafo"]. The

Published Data of Robert Munafo. . Retrieved 2011-1012.[6] Quark experiment predicts heavier Higgs (http:/ / www. newscientist. com/ article. ns?id=dn5095)[7] "New Theory Links Neutrino's Slight Mass To Accelerating Universe Expansion" (http:/ / www. sciencedaily. com/ releases/ 2004/ 07/

040728090338. htm). www.sciencedaily.com. . Retrieved 2008-06-05.

Further reading

General readers• Feynman, R.P. & Weinberg, S. (1987) Elementary Particles and the Laws of Physics: The 1986 Dirac Memorial

Lectures. Cambridge Univ. Press.• Ford, Kenneth W. (2005) The Quantum World. Harvard Univ. Press.• Brian Greene (1999). The Elegant Universe. W.W.Norton & Company. ISBN 0-393-05858-1.• John Gribbin (2000) Q is for Quantum - An Encyclopedia of Particle Physics. Simon & Schuster. ISBN

0-684-85578-X.• Oerter, Robert (2006) The Theory of Almost Everything: The Standard Model, the Unsung Triumph of Modern

Physics. Plume.• Schumm, Bruce A. (2004) Deep Down Things: The Breathtaking Beauty of Particle Physics. John Hopkins Univ.

Press. ISBN 0-8018-7971-X.• Martinus Veltman (2003). Facts and Mysteries in Elementary Particle Physics. World Scientific.

ISBN 981-238-149-X.• Frank Close (2004). Particle Physics: A Very Short Introduction. Oxford: Oxford University Press.

ISBN 0-19-280434-0.

Textbooks• Bettini, Alessandro (2008) Introduction to Elementary Particle Physics. Cambridge Univ. Press. ISBN

9780521880213• Coughlan, G. D., J. E. Dodd, and B. M. Gripaios (2006) The Ideas of Particle Physics: An Introduction for

Scientists, 3rd ed. Cambridge Univ. Press. An undergraduate text for those not majoring in physics.• Griffiths, David J. (1987) Introduction to Elementary Particles. John Wiley & Sons. ISBN 0-471-60386-4.• Kane, Gordon L. (1987). Modern Elementary Particle Physics. Perseus Books. ISBN 0-201-11749-5.• Perkins, Donald H. (2000) Introduction to High Energy Physics, 4th ed. Cambridge Univ. Press.

Elementary particle 32

External linksThe most important address about the current experimental and theoretical knowledge about elementary particlephysics is the Particle Data Group, where different international institutions collect all experimental data and giveshort reviews over the contemporary theoretical understanding.• Particle Data Group (http:/ / pdg. lbl. gov/ )other pages are:• Greene, Brian, " Elementary particles (http:/ / www. pbs. org/ wgbh/ nova/ elegant/ part-flash. html)", The

Elegant Universe, NOVA (PBS)• particleadventure.org (http:/ / particleadventure. org), a well-made introduction also for non physicists• CERNCourier: Season of Higgs and melodrama (http:/ / www. cerncourier. com/ main/ article/ 41/ 2/ 17)• Pentaquark information page (http:/ / plato. phy. ohiou. edu/ ~hicks/ thplus. htm)• Interactions.org (http:/ / www. interactions. org), particle physics news• Symmetry Magazine (http:/ / www. symmetrymagazine. org), a joint Fermilab/SLAC publication• "Sized Matter: perception of the extreme unseen" (http:/ / www-personal. umich. edu/ ~janhande/ sizedmatter/

sizedmatter_images. htm), Michigan University project for artistic visualisation of subatomic particles• Elementary Particles made thinkable (http:/ / www. thingsmadethinkable. com/ item/ elementary_particles. php),

an interactive visualisation allowing physical properties to be compared

BosonFor other meanings, see Boson (disambiguation).

The Standard Model of elementary particles, with the gauge bosons in the lastcolumn

In particle physics, bosons are subatomicparticles that obey Bose–Einstein statistics.Several bosons can occupy the samequantum state. The word boson derives fromthe name of Satyendra Nath Bose.[1]

Bosons contrast with fermions, which obeyFermi–Dirac statistics. Two or morefermions cannot occupy the same quantumstate.

Since bosons with the same energy canoccupy the same place in space, bosons areoften force carrier particles. In contrast,fermions are usually associated with matter(although in quantum physics the distinctionbetween the two concepts is not clear cut).

Bosons may be either elementary, likephotons, or composite, like mesons. Somecomposite bosons do not satisfy the criteriafor Bose-Einstein statistics and are not trulybosons (e.g. helium-4 atoms); a moreaccurate term for such composite particleswould be "bosonic-composites".

Boson 33

All observed bosons have integer spin, as opposed to fermions, which have half-integer spin. This is in accordancewith the spin-statistics theorem which states that in any reasonable relativistic quantum field theory, particles withinteger spin are bosons, while particles with half-integer spin are fermions.While most bosons are composite particles, in the Standard Model, there are six bosons which are elementary:• the four gauge bosons (γ · g · W± · Z)• the Higgs boson (H0)• the Graviton (G)Unlike the gauge bosons, the Higgs boson and Graviton have not yet been observed experimentally.[2]

Composite bosons are important in superfluidity and other applications of Bose–Einstein condensates.

Definition and basic propertiesBy definition, bosons are particles which obey Bose–Einstein statistics: when one swaps two bosons, thewavefunction of the system is unchanged.[3] Fermions, on the other hand, obey Fermi–Dirac statistics and the Pauliexclusion principle: two fermions cannot occupy the same quantum state, resulting in a "rigidity" or "stiffness" ofmatter which includes fermions. Thus fermions are sometimes said to be the constituents of matter, while bosons aresaid to be the particles that transmit interactions (force carriers), or the constituents of radiation. The quantum fieldsof bosons are bosonic fields, obeying canonical commutation relations.The properties of lasers and masers, superfluid helium-4 and Bose–Einstein condensates are all consequences ofstatistics of bosons. Another result is that the spectrum of a photon gas in thermal equilibrium is a Planck spectrum,one example of which is black-body radiation; another is the thermal radiation of the opaque early Universe seentoday as microwave background radiation. Interactions between elementary particles are called fundamentalinteractions. The fundamental interactions of virtual bosons with real particles result in all forces we know.All known elementary and composite particles are bosons or fermions, depending on their spin: particles withhalf-integer spin are fermions; particles with integer spin are bosons. In the framework of nonrelativistic quantummechanics, this is a purely empirical observation. However, in relativistic quantum field theory, the spin-statisticstheorem shows that half-integer spin particles cannot be bosons and integer spin particles cannot be fermions.[4]

In large systems, the difference between bosonic and fermionic statistics is only apparent at large densities—whentheir wave functions overlap. At low densities, both types of statistics are well approximated by Maxwell-Boltzmannstatistics, which is described by classical mechanics.

Elementary bosonsAll observed elementary particles are either fermions or bosons. The observed elementary bosons are all gaugebosons: photons, W and Z bosons and gluons.• Photons are the force carriers of the electromagnetic field.• W and Z bosons are the force carriers which mediate the weak force.• Gluons are the fundamental force carriers underlying the strong force.In addition, the standard model postulates the existence of Higgs bosons, which give other particles their mass viathe Higgs mechanism.Finally, many approaches to quantum gravity postulate a force carrier for gravity, the graviton, which is a boson ofspin 2.

Boson 34

Composite bosonsComposite particles (such as hadrons, nuclei, and atoms) can be bosons or fermions depending on their constituents.More precisely, because of the relation between spin and statistics, a particle containing an even number of fermionsis a boson, since it has integer spin.Examples include the following:• Any meson, since mesons contain one quark and one antiquark• The nucleus of a carbon-12 atom, which contains 6 protons and 6 neutrons• The helium-4 atom, consisting of 2 protons, 2 neutrons and 2 electronsThe number of bosons within a composite particle made up of simple particles bound with a potential has no effecton whether it is a boson or a fermion.Fermionic or bosonic behavior of a composite particle (or system) is only seen at large (compared to size of thesystem) distance. At proximity, where spatial structure begins to be important, a composite particle (or system)behaves according to its constituent makeup. For example, two atoms of helium-4 cannot share the same space if it iscomparable in size to that of the inner structure of the helium atom itself (~10−10 m)—despite bosonic properties ofthe helium-4 atoms. Thus, liquid helium has finite density comparable to the density of ordinary liquid matter.

Notes[1] "boson (dictionary entry)" (http:/ / www. merriam-webster. com/ dictionary/ boson). Merriam-Webster's Online Dictionary. . Retrieved

2010-03-21.[2] Standard Model of Particle Physics (http:/ / www-sldnt. slac. stanford. edu/ alr/ standard_model. htm), SLAC Large Detector (SLD) group

(http:/ / www-sld. slac. stanford. edu/ sldwww/ sld. html), Stanford Linear Accelerator Center (http:/ / www. slac. stanford. edu).[3] Srednicki (2007), pages 28-29[4] Sakurai (1994), page 362

References• Sakurai, J.J. (1994). Modern Quantum Mechanics (Revised Edition), pp 361–363. Addison-Wesley Publishing

Company, ISBN 0-201-53929-2.• Srednicki, Mark (2007). Quantum Field Theory (http:/ / www. physics. ucsb. edu/ ~mark/ qft. html), Cambridge

University Press, ISBN 978-0521864497.

Higgs boson 35

Higgs boson

Higgs boson

A simulated event, featuring the appearance of the Higgs bosonComposition Elementary particle

Statistics Bosonic

Status Hypothetical

Theorized F. Englert, R. Brout, P. Higgs, G. S. Guralnik, C. R. Hagen, and T. W. B. Kibble (1964)

Discovered Not yet (as of November 2011); searches ongoing at the LHC

Types 1, according to the Standard Model;5 or more, according to supersymmetric models

Mass 115–185 GeV/c2 (model-dependent upper bound[1] )

Spin 0

The Higgs boson is a hypothetical massive elementary particle that is predicted to exist by the Standard Model (SM)of particle physics. Its existence is postulated as a means of resolving inconsistencies in the Standard Model.Experiments attempting to find the particle are currently being performed using the Large Hadron Collider (LHC) atCERN, and were performed at Fermilab's Tevatron until Tevatron's closure in late 2011.The Higgs boson is the only elementary particle predicted by the Standard Model that has not been observed inparticle physics experiments. It is a necessary requirement of the so-called Higgs mechanism, the part of the SMwhich explains how most of the known elementary particles obtain their mass.[2] For example, the Higgs mechanismwould explain why the W and Z bosons, which mediate weak interactions, are massive whereas the related photon,which mediates electromagnetism, is massless. The Higgs boson is expected to be in a class of particles known asscalar bosons. (Bosons are particles with integer spin, and scalar bosons have spin 0.)Theories that do not need the Higgs boson are described as Higgsless models. Some theories suggest that anymechanism capable of generating the masses of the elementary particles must be visible at energies below 1.4TeV;[3] therefore, the LHC is expected to be able to provide experimental evidence of the existence or non-existenceof the Higgs boson.[4] Experiments at Fermilab were also continuing previous attempts at detection, albeit hinderedby the lower energy of the Tevatron accelerator. The Fermilab collider ceased operation 30th September 2011although data analysis still continues.

Higgs boson 36

Origin of the theory

Five of the six 2010 APS J.J. Sakurai Prizewinners. From left to right: Kibble, Guralnik,

Hagen, Englert, and Brout.

No. six: Peter Higgs 2009

The Higgs mechanism is a process by which vector bosons can get amass. It was proposed in 1964 independently and almostsimultaneously by three groups of physicists: François Englert andRobert Brout;[5] by Peter Higgs[6] (inspired by ideas of PhilipAnderson[7] ); and by Gerald Guralnik, C. R. Hagen, and TomKibble.[8]

The three papers written on this discovery were each recognized asmilestone papers during Physical Review Letters's 50th anniversarycelebration.[9] While each of these famous papers took similarapproaches, the contributions and differences between the 1964 PRLsymmetry breaking papers are noteworthy. These six physicists werealso awarded the 2010 J. J. Sakurai Prize for Theoretical ParticlePhysics for this work.[10]

The 1964 PRL papers by Higgs and by Guralnik, Hagen, and Kibble(GHK) both displayed equations for the field that would eventuallybecome known as the Higgs boson. In the paper by Higgs the boson ismassive, and in a closing sentence Higgs writes that "an essentialfeature" of the theory "is the prediction of incomplete multiplets ofscalar and vector bosons". In the model described in the GHK paperthe boson is massless and decoupled from the massive states. In recentreviews of the topic, Guralnik states that in the GHK model the bosonis massless only in a lowest-order approximation, but it is not subjectto any constraint and it acquires mass at higher orders. Additionally, he states that the GHK paper was the only oneto show that there are no massless Nambu-Goldstone bosons in the model and to give a complete analysis of thegeneral Higgs mechanism. [11] [12] Following the publication of the 1964 PRL papers, the properties of the modelwere further discussed by Guralnik in 1965 and by Higgs in 1966.[13] [14]

Steven Weinberg and Abdus Salam were the first to apply the Higgs mechanism to the electroweak symmetrybreaking. The Higgs mechanism not only explains how the electroweak vector bosons get a mass, but predicts theratio between the W boson and Z boson masses as well as their couplings with each other and with the StandardModel quarks and leptons. Many of these predictions have been verified by precise measurements performed at theLEP and the SLC colliders, thus confirming that the Higgs mechanism takes place in nature.[15]

The Higgs boson's existence is not a strictly necessary consequence of the Higgs mechanism: the Higgs boson existsin some but not all theories which use the Higgs mechanism. For example, the Higgs boson exists in the StandardModel and the Minimal Supersymmetric Standard Model yet is not expected to exist in Higgsless models, such asTechnicolor. A goal of the LHC and Tevatron experiments is to distinguish among these models and determine if theHiggs boson exists or not.

Higgs boson 37

Theoretical overview

A one-loop Feynman diagram of the first-ordercorrection to the Higgs mass. The Higgs boson couples

strongly to the top quark so it might decay intotop–anti-top quark pairs if it were heavy enough.

The Higgs boson particle is the quantum of the theoretical Higgsfield. In empty space, the Higgs field has an amplitude differentfrom zero; i.e. a non-zero vacuum expectation value. Theexistence of this non-zero vacuum expectation plays afundamental role; it gives mass to every elementary particle thatcouples to the Higgs field, including the Higgs boson itself. Theacquisition of a non-zero vacuum expectation value spontaneouslybreaks electroweak gauge symmetry. This is the Higgsmechanism, which is the simplest process capable of giving massto the gauge bosons while remaining compatible with gaugetheories. This field is analogous to a pool of molasses that "sticks"to the otherwise massless fundamental particles that travel through the field, converting them into particles with massthat form (for example) the components of atoms.

In the Standard Model, the Higgs field consists of two neutral and two charged component fields. Both of thecharged components and one of the neutral fields are Goldstone bosons, which act as the longitudinalthird-polarization components of the massive W+, W–, and Z bosons. The quantum of the remaining neutralcomponent corresponds to the massive Higgs boson. Since the Higgs field is a scalar field, the Higgs boson has nospin, hence no intrinsic angular momentum. The Higgs boson is also its own antiparticle and is CP-even.

The Standard Model does not predict the mass of the Higgs boson. If that mass is between 115 and 180 GeV/c2, thenthe Standard Model can be valid at energy scales all the way up to the Planck scale (1016 TeV). Many theoristsexpect new physics beyond the Standard Model to emerge at the TeV-scale, based on unsatisfactory properties of theStandard Model. The highest possible mass scale allowed for the Higgs boson (or some other electroweak symmetrybreaking mechanism) is 1.4 TeV; beyond this point, the Standard Model becomes inconsistent without such amechanism, because unitarity is violated in certain scattering processes. There are over a hundred theoreticalHiggs-mass predictions.[16]

Extensions to the Standard Model including supersymmetry (SUSY) predict the existence of families of Higgsbosons, rather than the one Higgs particle of the Standard Model. Among the SUSY models, in the MinimalSupersymmetric Standard Model (MSSM) the Higgs mechanism yields the smallest number of Higgs bosons: thereare two Higgs doublets, leading to the existence of a quintet of scalar particles, two CP-even neutral Higgs bosons hand H, a CP-odd neutral Higgs boson A, and two charged Higgs particles H±. Many supersymmetric models predictthat the lightest Higgs boson will have a mass only slightly above the current experimental limits, at around120 GeV/c2 or less.

Experimental search

Status as of March 2011, to the indicated confidence intervals

As of November 2011, the Higgsboson has yet to be confirmedexperimentally,[17] despite large effortsinvested in accelerator experiments atCERN and Fermilab.

Prior to the year 2000, the datagathered at the LEP collider at CERN

Higgs boson 38

A Feynman diagram of one way the Higgs boson maybe produced at the LHC. Here, two gluons convert totwo top/anti-top pairs, which then combine to make a

neutral Higgs.

A Feynman diagram of another way the Higgs bosonmay be produced at the LHC. Here, two quarks each

emit a W or Z boson, which combine to make a neutralHiggs.

allowed an experimental lower bound to be set for the mass of theStandard Model Higgs boson of 114.4 GeV/c2 at the 95%confidence level. The same experiment has produced a smallnumber of events that could be interpreted as resulting from Higgsbosons with a mass just above this cut off — around 115GeV—but the number of events was insufficient to draw definiteconclusions.[18] The LEP was shut down in 2000 due toconstruction of its successor, the LHC, which is expected to beable to confirm or reject the existence of the Higgs boson. Fulloperational mode was delayed until mid-November 2009, becauseof a serious fault discovered with a number of magnets during thecalibration and startup phase.[19] [20]

At the Fermilab Tevatron, there are ongoing experimentssearching for the Higgs boson. As of July 2010, combined datafrom CDF and DØ experiments at the Tevatron were sufficient toexclude the Higgs boson in the range 158 GeV/c2 - 175 GeV/c2 atthe 95% confidence level.[21] [22] Preliminary results as of July2011 have since extended the excluded region to the range156 GeV/c2 - 177 GeV/c2 at the 90% confidence level.[23] Datacollection and analysis in search of Higgs are intensifying sinceMarch 30, 2010 when the LHC began operating at 3.5 TeV.[24]

Preliminary results from the ATLAS and CMS experiments at theLHC as of July 2011 exclude a Standard Model Higgs boson in themass range 155 GeV/c2 - 190 GeV/c2[25] and 149 GeV/c2 -206 GeV/c2,[26] respectively, at the 95% confidence level.

It may be possible to estimate the mass of the Higgs boson indirectly. In the Standard Model, the Higgs boson has anumber of indirect effects; most notably, Higgs loops result in tiny corrections to masses of W and Z bosons.Precision measurements of electroweak parameters, such as the Fermi constant and masses of W/Z bosons, can beused to constrain the mass of the Higgs. As of 2006, measurements of electroweak observables allowed the exclusionof a Standard Model Higgs boson having a mass greater than 285 GeV/c2 at 95% CL, and estimated its mass to be129  GeV/c2 (the central value corresponding to approximately 138 proton masses).[27] As of August 2009, theStandard Model Higgs boson is excluded by electroweak measurements above 186 GeV at the 95% confidence level.However, it should be noted that these indirect constraints make the assumption that the Standard Model is correct. Itmay still be possible to discover a Higgs boson above 186 GeV if it is accompanied by other particles between theStandard Model and GUT scales.In a 2009 preprint,[28] [29] it was suggested that the Higgs boson might not only interact with the above-mentionedparticles of the Standard model of particle physics, but also with the mysterious weakly interacting massive particles(or WIMPS) that may form dark matter, and which play an important role in recent astrophysics.Various reports of potential evidence for the existence of the Higgs boson have appeared in recent years[30] [31] [32]

but to date none have provided convincing evidence. In April 2011, there were suggestions in the media that evidence for the Higgs boson might have been discovered at the LHC in Geneva, Switzerland[33] but these had been debunked by mid May.[34] In regard to these rumors Jon Butterworth, a member of the High Energy Physics group

Higgs boson 39

on the Atlas experiment, stated they were not a hoax, but were based on unofficial, unreviewed results.[35] The LHCdetected possible signs of the particle, which were reported in July 2011, the ATLAS Note concluding: "In the lowmass range (c 120−140 GeV) an excess of events with a significance of approximately 2.8 sigma above thebackground expectation is observed" and the BBC reporting that "interesting particle events at a mass of between140 and 145 GeV" were found.[36] [37] These findings were repeated shortly thereafter by researchers at the Tevatronwith a spokesman stating that: "There are some intriguing things going on around a mass of 140GeV."[36] However,on 22 August it was reported that the anomalous results had become insignificant on the inclusion of more data fromATLAS and CMS and that the non-existence of the particle had been confirmed by LHC collisions to 95% certaintybetween 145–466 GeV (except for a few small islands around 250 GeV).[38] A combined analysis of ATLAS andCMS data, published in November 2011, further narrowed the window for the allowed values of the Higgs bosonmass to 114-141 GeV.[39]

Alternatives for electroweak symmetry breakingIn the years since the Higgs boson was proposed, several alternatives to the Higgs mechanism have been proposed.All of these proposed mechanisms use strongly interacting dynamics to produce a vacuum expectation value thatbreaks electroweak symmetry. A partial list of these alternative mechanisms are:• Technicolor,[40] a class of models that attempts to mimic the dynamics of the strong force as a way of breaking

electroweak symmetry.• Extra dimensional Higgsless models where the role of the Higgs field is played by the fifth component of the

gauge field.[41]

• Abbott-Farhi models of composite W and Z vector bosons.[42]

• Top quark condensate theory in which a fundamental scalar Higgs field is replaced by a composite fieldcomposed of the top quark and its antiquark.

• The braid model of Standard Model particles by Sundance Bilson-Thompson, compatible with loop quantumgravity and similar theories.[43]

• In theory of superfluid vacuum masses of elementary particles can arise as a result of interaction with the physicalvacuum, similarly to the gap generation mechanism in superconductors.[44] [45]

"The God particle"The Higgs boson is often referred to as "the God particle" by the media,[46] after the title of Leon Lederman's book,The God Particle: If the Universe Is the Answer, What Is the Question?[47] Lederman initially wanted to call it the"goddamn particle," but his editor would not let him.[48] While use of this term may have contributed to increasedmedia interest in particle physics and the Large Hadron Collider,[47] many scientists dislike it, since it overstates theparticle's importance, not least since its discovery would still leave unanswered questions about the unification ofQCD, the electroweak interaction and gravity, and the ultimate origin of the universe.[46] A renaming competitionconducted by the science correspondent for the British Guardian newspaper chose the name "the champagne bottleboson" as the best from among their submissions: "The bottom of a champagne bottle is in the shape of the Higgspotential and is often used as an illustration in physics lectures. So it's not an embarrassingly grandiose name, it ismemorable, and [it] has some physics connection too."[49]

Higgs boson 40

Notes[1] This upper bound for the Higgs boson mass is a prediction within the minimal Standard Model assuming that it remains a consistent theory up

to the Planck scale. In extensions of the SM, this bound can be loosened or, in the case of supersymmetry theories, lowered. The lower boundwhich results from direct experimental exclusion by LEP is valid for most extensions of the SM, but can be circumvented in special cases.(http:/ / pdg. lbl. gov/ 2010/ reviews/ rpp2010-rev-higgs-boson. pdf)

[2] The masses of composite particles such as the proton and neutron would only be partly due to the Higgs mechanism, and are alreadyunderstood as a consequence of the strong interaction.

[3] Lee, Benjamin W.; Quigg, C.; Thacker, H. B. (1977). "Weak interactions at very high energies: The role of the Higgs-boson mass". PhysicalReview D 16 (5): 1519–1531. Bibcode 1977PhRvD..16.1519L. doi:10.1103/PhysRevD.16.1519.

[4] "Huge $10 billion collider resumes hunt for 'God particle' - CNN.com" (http:/ / www. cnn. com/ 2009/ TECH/ 11/ 11/ lhc. large. hadron.collider. beam/ index. html). CNN. 2009-11-11. . Retrieved 2010-05-04.

[5] Englert, François; Brout, Robert (1964). "Broken Symmetry and the Mass of Gauge Vector Mesons". Physical Review Letters 13 (9): 321–23.Bibcode 1964PhRvL..13..321E. doi:10.1103/PhysRevLett.13.321.

[6] Higgs, Peter (1964). "Broken Symmetries and the Masses of Gauge Bosons". Physical Review Letters 13 (16): 508–509.Bibcode 1964PhRvL..13..508H. doi:10.1103/PhysRevLett.13.508.

[7] Ph. Anderson: "Plasmons, gauge invariance and mass." In: Physical Review. 130, 1963, p. 439–442[8] Guralnik, Gerald; Hagen, C. R.; Kibble, T. W. B. (1964). "Global Conservation Laws and Massless Particles". Physical Review Letters 13

(20): 585–587. Bibcode 1964PhRvL..13..585G. doi:10.1103/PhysRevLett.13.585.[9] Physical Review Letters - 50th Anniversary Milestone Papers (http:/ / prl. aps. org/ 50years/ milestones#1964). Physical Review Letters. .[10] "American Physical Society - J. J. Sakurai Prize Winners" (http:/ / www. aps. org/ units/ dpf/ awards/ sakurai. cfm). .[11] G.S. Guralnik (2009). "The History of the Guralnik, Hagen and Kibble development of the Theory of Spontaneous Symmetry Breaking and

Gauge Particles". International Journal of Modern Physics A 24 (14): 2601–2627. arXiv:0907.3466. Bibcode 2009IJMPA..24.2601G.doi:10.1142/S0217751X09045431.

[12] "Guralnik, G.S. The Beginnings of Spontaneous Symmetry Breaking in Particle Physics. Proceedings of the DPF-2011 Conference,Providence, RI, August 8–13, 2011" (http:/ / arxiv. org/ abs/ 1110. 2253v1). Arxiv.org. 2011-10-11. . Retrieved 2011-12-07.

[13] G.S. Guralnik (2011). "GAUGE INVARIANCE AND THE GOLDSTONE THEOREM - 1965 Feldafing talk". Modern Physics Letters A26 (19): 1381–1392. arXiv:1107.4592v1. Bibcode 2011MPLA...26.1381G. doi:10.1142/S0217732311036188.

[14] Higgs, Peter (1966). "Spontaneous Symmetry Breakdown without Massless Bosons". Physical Review 145 (4): 1156–1163.Bibcode 1966PhRv..145.1156H. doi:10.1103/PhysRev.145.1156.

[15] "LEP Electroweak Working Group" (http:/ / lepewwg. web. cern. ch/ LEPEWWG/ ). .[16] T. Schücker (2007). "Higgs-mass predictions". arXiv:0708.3344 [hep-ph].[17] Scientists present first “bread-and-butter” results from LHC collisions (http:/ / www. symmetrymagazine. org/ breaking/ 2010/ 06/ 08/

scientists-present-first-bread-and-butter-results-from-lhc-collisions/ ) Symmetry Breaking, 8 June 2010[18] W.-M. Yao et al. (2006). Searches for Higgs Bosons "Review of Particle Physics" (http:/ / pdg. lbl. gov/ 2006/ reviews/ higgs_s055. pdf).

Journal of Physics G 33: 1. arXiv:astro-ph/0601168. Bibcode 2006JPhG...33....1Y. doi:10.1088/0954-3899/33/1/001. Searches for HiggsBosons.

[19] "CERN management confirms new LHC restart schedule" (http:/ / press. web. cern. ch/ press/ PressReleases/ Releases2009/ PR02. 09E.html). CERN Press Office. 9 February 2009. . Retrieved 2009-02-10.

[20] "CERN reports on progress towards LHC restart" (http:/ / press. web. cern. ch/ press/ PressReleases/ Releases2009/ PR09. 09E. html).CERN Press Office. 19 June 2009. . Retrieved 2009-07-21.

[21] T. Aaltonen et al. (CDF and DØ Collaborations) (2010). "Combination of Tevatron searches for the standard model Higgs boson in theW+W− decay mode". Physical Review Letters 104 (6). arXiv:1001.4162. Bibcode 2010PhRvL.104f1802A.doi:10.1103/PhysRevLett.104.061802.

[22] "Fermilab experiments narrow allowed mass range for Higgs boson" (http:/ / www. fnal. gov/ pub/ presspass/ press_releases/Higgs-mass-constraints-20100726-images. html). Fermilab. 26 July 2010. . Retrieved 2010-07-26.

[23] The CDF & D0 Collaborations (27 July 2011). "Combined CDF and D0 Upper Limits on Standard Model Higgs Boson Production with upto 8.6 fb-1 of Data". arXiv:1107.5518 [hep-ex].

[24] "''CERN Bulletin'' Issue No. 18-20/2010 - Monday 3 May 2010" (http:/ / cdsweb. cern. ch/ journal/ CERNBulletin/ 2010/ 18/ News Articles/1262593?ln=en). Cdsweb.cern.ch. 2010-05-03. . Retrieved 2011-12-07.

[25] "Combined Standard Model Higgs Boson Searches in pp Collisions at root-s = 7 TeV with the ATLAS Experiment at the LHC" (https:/ /atlas. web. cern. ch/ Atlas/ GROUPS/ PHYSICS/ CONFNOTES/ ATLAS-CONF-2011-112/ ). 24 July 2011. ATLAS-CONF-2011-112. .

[26] "Search for standard model Higgs boson in pp collisions at sqrt{s}=7 TeV" (http:/ / cdsweb. cern. ch/ record/ 1370076/ ). 23 July 2011.CMS-PAS-HIG-11-011. .

[27] " H0 Indirect Mass Limits from Electroweak Analysis. (http:/ / pdglive. lbl. gov/ popupblockdata. brl?nodein=S055HEW& inscript=Y&fsizein=1)"

[28] C. B. Jackson; Geraldine Servant; Gabe Shaughnessy; Tim M. P. Tait; Marco Taoso (2009). "Higgs in Space!". arXiv:0912.0004 [hep-ph].[29] Physics World, "Higgs could reveal itself in Dark-Matter collisions (http:/ / physicsworld. com/ cws/ article/ news/ 41218). British Institute

of Physics. Retrieved 26 July 2011.

Higgs boson 41

[30] Potential Higgs Boson discovery: " Higgs Boson: Glimpses of the God particle. (http:/ / www. newscientist. com/ channel/ fundamentals/mg19325934. 600-higgs-boson-glimpses-of-the-god-particle. html)" New Scientist, 02 March 2007

[31] " 'God particle' may have been seen, (http:/ / news. bbc. co. uk/ 2/ hi/ science/ nature/ 3546973. stm)" BBC news, 10 March 2004.[32] US experiment hints at 'multiple God particles' (http:/ / news. bbc. co. uk/ 1/ hi/ science_and_environment/ 10313875. stm) BBC News 14

June 2010[33] "Mass hysteria! Science world buzzing over rumours the elusive 'God Particle' has finally been found- dailymail.co.uk" (http:/ / www.

dailymail. co. uk/ sciencetech/ article-1379844/ Science-world-buzzing-rumours-elusive-God-particle-found. html). Mail Online. 2011-04-24.. Retrieved 2011-04-24.

[34] The Collider That Cried Higgs (http:/ / www. nature. com/ news/ 2011/ 110510/ full/ 473136a. html) Nature 473, 136-137 (2011)[35] Butterworth, Jon (2011-04-24). "The Guardian, "Rumours of the Higgs at ATLAS"" (http:/ / www. guardian. co. uk/ science/

life-and-physics/ 2011/ apr/ 24/ 1?CMP=twt_fd). Guardian. . Retrieved 2011-12-07.[36] Rincon, Paul (24 July 2011) "Higgs boson 'hints' also seen by US lab" (http:/ / www. bbc. co. uk/ news/ science-environment-14266358)

BBC News. Retrieved 24 July 2011.[37] "Combined Standard Model Higgs Boson Searches in pp Collisions at √s = 7 TeV with the ATLAS Experiment at the LHC" (https:/ / atlas.

web. cern. ch/ Atlas/ GROUPS/ PHYSICS/ CONFNOTES/ ATLAS-CONF-2011-112/ ATLAS-CONF-2011-112. pdf) ATLAS Note (24 July2011) (pdf) The ATLAS Collaboration. Retrieved 26 July 2011.

[38] Ghosh, Pallab (2011-08-22). "BBC News - Higgs boson range narrows at European collider" (http:/ / www. bbc. co. uk/ news/science-environment-14596367). Bbc.co.uk. . Retrieved 2011-11-07.

[39] Geoff Brumfiel (2011-11-18). "Higgs hunt enters endgame" (http:/ / www. nature. com/ news/ higgs-hunt-enters-endgame-1. 9399). NatureNews. . Retrieved 2011-11-22.

[40] S. Dimopoulos and Leonard Susskind (1979). "Mass Without Scalars". Nuclear Physics B 155: 237–252. Bibcode 1979NuPhB.155..237D.doi:10.1016/0550-3213(79)90364-X.

[41] C. Csaki and C. Grojean and L. Pilo and J. Terning (2004). "Towards a realistic model of Higgsless electroweak symmetry breaking".Physical Review Letters 92 (10): 101802. arXiv:hep-ph/0308038. Bibcode 2004PhRvL..92j1802C. doi:10.1103/PhysRevLett.92.101802.PMID 15089195.

[42] L. F. Abbott and E. Farhi (1981). "Are the Weak Interactions Strong?". Physics Letters B 101: 69. Bibcode 1981PhLB..101...69A.doi:10.1016/0370-2693(81)90492-5.

[43] Bilson-Thompson, Sundance O.; Markopoulou, Fotini; Smolin, Lee (2007). "Quantum gravity and the standard model". Class. QuantumGrav. 24 (16): 3975–3993. arXiv:hep-th/0603022. Bibcode 2007CQGra..24.3975B. doi:10.1088/0264-9381/24/16/002.

[44] K. G. Zloshchastiev, Spontaneous symmetry breaking and mass generation as built-in phenomena in logarithmic nonlinear quantum theory,Acta Phys. Polon. B 42 (2011) 261-292 ArXiv:0912.4139 (http:/ / arxiv. org/ abs/ 0912. 4139).

[45] A. V. Avdeenkov and K. G. Zloshchastiev, Quantum Bose liquids with logarithmic nonlinearity: Self-sustainability and emergence ofspatial extent, J. Phys. B: At. Mol. Opt. Phys. 44 (2011) 195303. ArXiv:1108.0847 (http:/ / arxiv. org/ abs/ 1108. 0847).

[46] Ian Sample (29 May 2009). "Anything but the God particle" (http:/ / www. guardian. co. uk/ science/ blog/ 2009/ may/ 29/why-call-it-the-god-particle-higgs-boson-cern-lhc). London: The Guardian. . Retrieved 2009-06-24.

[47] Ian Sample (3 March 2009). "Father of the God particle: Portrait of Peter Higgs unveiled" (http:/ / www. guardian. co. uk/ science/ blog/2009/ mar/ 02/ god-particle-peter-higgs-portrait-lhc). London: The Guardian. . Retrieved 2009-06-24.

[48] Randerson, James (June 30, 2008). "Father of the 'God Particle'" (http:/ / www. guardian. co. uk/ science/ 2008/ jun/ 30/ higgs. boson. cern).The Guardian. .

[49] Ian Sample (12 June 2009). "Higgs competition: Crack open the bubbly, the God particle is dead" (http:/ / www. guardian. co. uk/ science/blog/ 2009/ jun/ 05/ cern-lhc-god-particle-higgs-boson). The Guardian (London). . Retrieved 2010-05-04.

References

Further reading• G.S. Guralnik, C.R. Hagen and T.W.B. Kibble (1964). "Global Conservation Laws and Massless Particles".

Physical Review Letters 13 (20): 585. Bibcode 1964PhRvL..13..585G. doi:10.1103/PhysRevLett.13.585.• G.S. Guralnik (2009). "The History of the Guralnik, Hagen and Kibble development of the Theory of

Spontaneous Symmetry Breaking and Gauge Particles". International Journal of Modern Physics A 24 (14):2601–2627. arXiv:0907.3466. Bibcode 2009IJMPA..24.2601G. doi:10.1142/S0217751X09045431.

• Guralnik, G S; Hagen, C R and Kibble, T W B (1967). Broken Symmetries and the Goldstone Theorem.Advances in Physics, vol. 2 (http:/ / www. datafilehost. com/ download-7d512618. html)

• F. Englert and R. Brout (1964). "Broken Symmetry and the Mass of Gauge Vector Mesons". Physical ReviewLetters 13 (9): 321. Bibcode 1964PhRvL..13..321E. doi:10.1103/PhysRevLett.13.321.

Higgs boson 42

• P. Higgs (1964). "Broken Symmetries, Massless Particles and Gauge Fields". Physics Letters 12 (2): 132.Bibcode 1964PhL....12..132H. doi:10.1016/0031-9163(64)91136-9.

• P. Higgs (1964). "Broken Symmetries and the Masses of Gauge Bosons". Physical Review Letters 13 (16): 508.Bibcode 1964PhRvL..13..508H. doi:10.1103/PhysRevLett.13.508.

• P. Higgs (1966). "Spontaneous Symmetry Breakdown without Massless Bosons". Physical Review 145 (4): 1156.Bibcode 1966PhRv..145.1156H. doi:10.1103/PhysRev.145.1156.

• Y. Nambu and G. Jona-Lasinio (1961). "Dynamical Model of Elementary Particles Based on an Analogy withSuperconductivity". Physical Review 122: 345–358. Bibcode 1961PhRv..122..345N.doi:10.1103/PhysRev.122.345.

• J. Goldstone, A. Salam and S. Weinberg (1962). "Broken Symmetries". Physical Review 127 (3): 965.Bibcode 1962PhRv..127..965G. doi:10.1103/PhysRev.127.965.

• P.W. Anderson (1963). "Plasmons, Gauge Invariance, and Mass". Physical Review 130: 439.Bibcode 1963PhRv..130..439A. doi:10.1103/PhysRev.130.439.

• A. Klein and B.W. Lee (1964). "Does Spontaneous Breakdown of Symmetry Imply Zero-Mass Particles?".Physical Review Letters 12 (10): 266. Bibcode 1964PhRvL..12..266K. doi:10.1103/PhysRevLett.12.266.

• W. Gilbert (1964). "Broken Symmetries and Massless Particles". Physical Review Letters 12 (25): 713.Bibcode 1964PhRvL..12..713G. doi:10.1103/PhysRevLett.12.713.

External links• Hunting the Higgs boson at C.M.S. Experiment, at CERN (http:/ / cmsinfo. web. cern. ch/ cmsinfo/ Physics/

HuntingHiggs/ index. html)• The Higgs boson (http:/ / www. exploratorium. edu/ origins/ cern/ ideas/ higgs. html)" by the CERN

exploratorium.• Particle Data Group: Review of searches for Higgs bosons. (http:/ / pdg. lbl. gov/ 2005/ reviews/ contents_sports.

html#hyppartetc)

Higgs mechanism 43

Higgs mechanismIn particle physics, the Higgs mechanism is the process in which gauge bosons in a gauge theory can acquirenon-vanishing masses through absorption of Nambu-Goldstone bosons arising in spontaneous symmetry breaking.The simplest implementation of the mechanism adds an extra Higgs field to the gauge theory. The spontaneoussymmetry breaking of the underlying local symmetry triggers conversion of components of this Higgs field toGoldstone bosons which interact with (at least some of) the other fields in the theory, so as to produce mass terms for(at least some of) the gauge bosons. This mechanism may also leave behind elementary scalar (spin-0) particles,known as Higgs bosons.In the standard model, the phrase "Higgs mechanism" refers specifically to the generation of masses for the W±, andZ weak gauge bosons through electroweak symmetry breaking.[1] Although the evidence for the electroweak Higgsmechanism is overwhelming, experiments have yet to discover the single Higgs boson predicted by the standardmodel. The Large Hadron Collider at CERN is currently searching for Higgs bosons, and attempting to understandthe electroweak Higgs mechanism.

In the standard modelThe Higgs mechanism was incorporated into modern particle physics by Steven Weinberg and Abdus Salam, and isan essential part of the standard model.In the standard model, at temperatures high enough so that electroweak symmetry is unbroken, all elementaryparticles are massless. At a critical temperature, the symmetry is spontaneously broken, and the W and Z bosonsacquire masses. (EWSB, ElectroWeak Symmetry Breaking, is an abbreviation used for this).Fermions, such as the leptons and quarks in the Standard Model, can also acquire mass as a result of their interactionwith the Higgs field, but not in the same way as the gauge bosons.

Structure of the Higgs fieldIn the standard model, the Higgs field is an SU(2) doublet, a complex spinor with four real components (orequivalently with two complex components), with a Standard Model U(1) charge of −1.It transforms as a spinor under SU(2). Under U(1) rotations, it gets multiplied by a phase; this mixes the real andimaginary part of the complex spinor into each other, so this is not the same as two complex spinors mixing underU(1) (which would have eight real components between them), but instead is the spinor representation of the groupU(2).The Higgs field, through the interactions specified by its potential, induces spontaneous breaking of three out of thefour generators ("directions") of the gauge group SU(2)×U(1), and three out of its four components would ordinarilyamount to Goldstone bosons, if they were not coupled to gauge fields. However, after symmetry breaking, thesethree of the four degrees of freedom in the Higgs field mix with the W and Z bosons, while the one remaining degreeof freedom becomes the Higgs boson – a new scalar particle.

The part that remains masslessThe gauge group of the electroweak part of the standard model is SU(2) × U(1).The group SU(2) is all unitary matrices, all the orthonormal changes of coordinates in a complex two dimensionalvector space. Rotating the coordinates so that the first basis vector in the direction of H makes the vacuumexpectation value of H the spinor (A, 0). The generators for rotations about the x, y, and z axes are by half the Paulimatrices , so that a rotation of angle θ about the z-axis takes the vacuum to:

Higgs mechanism 44

While the X and Y generators mix up the top and bottom components of the spinor, the Z rotations only multiply bya phase. This phase can be undone by a U(1) rotation of angle ½θ, which multiplies by the opposite phase, since theHiggs has charge −1. Under both an SU(2) z-rotation and a U(1) rotation by an amount ½θ, the vacuum is invariant.This combination of generators:

defines the unbroken gauge group, where Wz is the generator of rotations around the z-axis in the SU(2) and V is thegenerator of the U(1). This combination of generators (a z rotation in the SU(2) and a simultaneous U(1) rotation byhalf the angle) preserves the vacuum, and defines the unbroken gauge group in the standard model. The part of thegauge field in this direction stays massless, and this gauge field is the actual photon.The phase that a field acquires under this combination of generators is its electric charge, and this is the formula forthe electric charge in the standard model. In this convention, all the V charges in the standard model are multiples of⅓. To make all the V-charges in the standard model integers, you can rescale the V part of the formula by tripling allthe V-charges if you like, and rewrite the charge formula as:

but the normalization with ½V is the universal standard.

Consequences for fermionsIn spite of the introduction of spontaneous symmetry-breaking, also for fermions the mass terms oppose the chiralgauge invariance. Therefore, also for these fields the mass terms should be replaced by a gauge-invariant "Higgs"mechanism. An obvious possibility is some kind of "Yukawa coupling" (see below) between the fermion field ψ andthe Higgs field Φ, with unknown couplings , which after symmetry-breaking (more precisely: after expansionof the Lagrange density around a suitable ground state) again results in the original mass terms, which are now,however (i.e. by introduction of the Higgs field) written in a gauge-invariant way. The Lagrange density for the"Yukawa"-interaction of a fermion field 'ψ' and the Higgs field 'Φ' is

where again the gauge field A only enters (i.e., it is only indirectly visible). The quantities are the Diracmatrices, and is the already-mentioned "Yukawa"-coupling parameter. Already now the mass-generationfollows the same principle as above, namely from the existence of a finite expectation value , as describedabove. Again, this is crucial for the existence of the property "mass".

History of research

BackgroundSpontaneous symmetry breaking offered a framework to introduce bosons into relativistic quantum field theories.However, according to Goldstone's theorem, these bosons should be massless.[2] The only observed particles whichcould be approximately interpreted as Goldstone bosons were the pions, which Yoichiro Nambu related to chiralsymmetry breaking.A similar problem arises with Yang–Mills theory (also known as nonabelian gauge theory), which predicts masslessspin-1 gauge bosons. Massless weakly interacting gauge bosons lead to long-range forces, which are only observedfor electromagnetism and the corresponding massless photon. Gauge theories of the weak force needed a way todescribe massive gauge bosons in order to be consistent.

Higgs mechanism 45

Discovery

Five of the six 2010 APS Sakurai PrizeWinners- (L to R) Kibble, Guralnik,

Hagen, Englert, and Brout

Number six: P. Higgs 2009

The Higgs mechanism is also called the Brout–Englert–Higgs mechanism,or Englert-Brout-Higgs-Guralnik-Hagen-Kibble mechanism,[3] orAnderson–Higgs mechanism. The mechanism was proposed in 1962 byPhilip Warren Anderson,[4] who discussed its consequences for particlephysics but did not work out an explicit relativistic model. The relativisticmodel was developed in 1964 by Peter Higgs,[5] and independently by RobertBrout and Francois Englert,[6] and Gerald Guralnik, C. R. Hagen, and TomKibble,[7] who worked out the results by the spring of 1963.[8] Themechanism is closely analogous to phenomena previously discovered byYoichiro Nambu involving the "vacuum structure" of quantum fields insuperconductivity.[9] A similar but distinct effect, known as the Stueckelbergmechanism, had previously been studied by Ernst Stueckelberg. Thesephysicists discovered that when a gauge theory is combined with anadditional field breaking spontaneously the symmetry group, the gaugebosons can consistently acquire a finite mass. In spite of the large valuesinvolved (see below) this permits a gauge theory description of the weakforce, which was independently developed by Steven Weinberg and AbdusSalam in 1967. Higgs's original article presenting the model was rejected byPhysics Letters. When revising the article before resubmitting it to PhysicalReview Letters, he added a sentence at the end,[10] mentioning that it impliesthe existence of one or more new, massive scalar bosons, which do not form complete representations of thesymmetry group; these are the Higgs bosons.

The three papers by Guralnik, Hagen, and Kibble; Higgs; and Brout and Englert were each recognized as "milestoneletters" by Physical Review Letters in 2008.[11] While each of these seminal papers took similar approaches, thecontributions and differences among the 1964 PRL symmetry breaking papers are noteworthy. All six physicistswere jointly awarded the 2010 J. J. Sakurai Prize for Theoretical Particle Physics for this work.[12]

Benjamin W. Lee is often credited with first naming the "Higgs-like" mechanism, although there is debate aroundwhen this first occurred.[13] [14] [15] One of the first times the Higgs name appeared in print was in 1972 whenGerardus 't Hooft and Martinus J. G. Veltman referred to it as the "Higgs-Kibble mechanism" in their Nobel winningpaper.[16] [17]

Examples of Higgs mechanismThe Higgs mechanism occurs whenever a charged field has a vacuum expectation value. In the nonrelativisticcontext, this is the Landau model of a charged Bose-Einstein condensate, also known as a superconductor. In therelativistic condensate, the condensate is a scalar field, and is relativistically invariant.

Landau ModelThe Higgs mechanism is a type of superconductivity which occurs in the vacuum. It occurs when all of space isfilled with a sea of particles which are charged, or, in field language, when a charged field has a nonzero vacuumexpectation value. Interaction with the quantum fluid filling the space prevents certain forces from propagating overlong distances (as it does in a superconducting medium, e.g. in the Ginzburg–Landau theory).A superconductor expels all magnetic fields from its interior, a phenomenon known as the Meissner effect. This was mysterious for a long time, because it implies that electromagnetic forces somehow become short-range inside the superconductor. Contrast this with the behavior of an ordinary metal. In a metal, the conductivity shields electric

Higgs mechanism 46

fields by rearranging charges on the surface until the total field cancels in the interior. But magnetic fields canpenetrate to any distance, and if a magnetic monopole (an isolated magnetic pole) is surrounded by a metal the fieldcan escape without collimating into a string. In a superconductor, however, electric charges move with nodissipation, and this allows for permanent surface currents, not just surface charges. When magnetic fields areintroduced at the boundary of a superconductor, they produce surface currents which exactly neutralize them. TheMeissner effect is due to currents in a thin surface layer, whose thickness, the London penetration depth, can becalculated from a simple model (the Ginzburg–Landau theory).This simple model, treats superconductivity as a charged Bose–Einstein condensate. Suppose that a superconductorcontains bosons with charge q. The wavefunction of the bosons can be described by introducing a quantum field, ψ,which obeys the Schrödinger equation as a field equation (in units where , the Planck quantum divided by 2π, isreplaced by 1):

The operator ψ(x) annihilates a boson at the point x, while its adjoint creates a new boson at the same point. Thewavefunction of the Bose–Einstein condensate is then the expectation value ψ of ψ(x), which is a classical functionthat obeys the same equation. The interpretation of the expectation value is that it is the phase that one should give toa newly created boson so that it will coherently superpose with all the other bosons already in the condensate.When there is a charged condensate, the electromagnetic interactions are screened. To see this, consider the effect ofa gauge transformation on the field. A gauge transformation rotates the phase of the condensate by an amount whichchanges from point to point, and shifts the vector potential by a gradient.

When there is no condensate, this transformation only changes the definition of the phase of ψ at every point. Butwhen there is a condensate, the phase of the condensate defines a preferred choice of phase.The condensate wave function can be written as

where ρ is real amplitude, which determines the local density of the condensate. If the condensate were neutral, theflow would be along the gradients of θ, the direction in which the phase of the Schrödinger field changes. If thephase θ changes slowly, the flow is slow and has very little energy. But now θ can be made equal to zero just bymaking a gauge transformation to rotate the phase of the field.The energy of slow changes of phase can be calculated from the Schrödinger kinetic energy,

and taking the density of the condensate ρ to be constant,

Fixing the choice of gauge so that the condensate has the same phase everywhere, the electromagnetic field energyhas an extra term,

When this term is present, electromagnetic interactions become short-ranged. Every field mode, no matter how longthe wavelength, oscillates with a nonzero frequency. The lowest frequency can be read off from the energy of a longwavelength A mode,

Higgs mechanism 47

This is a harmonic oscillator with frequency:

The quantity |ψ|2 (=ρ2) is the density of the condensate of superconducting particles.In an actual superconductor, the charged particles are electrons, which are fermions not bosons. So in order to havesuperconductivity, the electrons need to somehow bind into Cooper pairs. The charge of the condensate q is thereforetwice the electron charge e. The pairing in a normal superconductor is due to lattice vibrations, and is in fact veryweak; this means that the pairs are very loosely bound. The description of a Bose–Einstein condensate of looselybound pairs is actually more difficult than the description of a condensate of elementary particles, and was onlyworked out in 1957 by Bardeen, Cooper and Schrieffer in the famous BCS theory.

Abelian Higgs MechanismGauge invariance means that certain transformations of the gauge field do not change the energy at all. If an arbitrarygradient is added to A, the energy of the field is exactly the same. This makes it difficult to add a mass term, becausea mass term tends to push the field toward the value zero. But the zero value of the vector potential is not a gaugeinvariant idea. What is zero in one gauge is nonzero in another.So in order to give mass to a gauge theory, the gauge invariance must be broken by a condensate. The condensatewill then define a preferred phase, and the phase of the condensate will define the zero value of the field in a gaugeinvariant way. The gauge invariant definition is that a gauge field is zero when the phase change along any path fromparallel transport is equal to the phase difference in the condensate wavefunction.The condensate value is described by a quantum field with an expectation value, just as in the Landau–Ginzburgmodel.In order for the phase of the vacuum to define a gauge, the field must have a phase (also referred to as 'to becharged'). In order for a scalar field Φ to have a phase, it must be complex, or (equivalently) it should contain twofields with a symmetry which rotates them into each other. The vector potential changes the phase of the quantaproduced by the field when they move from point to point. In terms of fields, it defines how much to rotate the realand imaginary parts of the fields into each other when comparing field values at nearby points.The only renormalizable model where a complex scalar field Φ acquires a nonzero value is the Mexican-hat model,where the field energy has a minimum away from zero.

This defines the following Hamiltonian:

The first term is the kinetic energy of the field. The second term is the extra potential energy when the field variesfrom point to point. The third term is the potential energy when the field has any given magnitude.This potential energy V(z, Φ) = λ • (|z|2 - Φ2)2 has a graph which looks like a Mexican hat, which gives the model itsname. In particular, the minimum energy value is not at z = 0, but on the circle of points where the magnitude of z isΦ.

Higgs mechanism 48

Higgs potential V. For a fixed value of λ thepotential is presented against the real andimaginary parts of Φ. The Mexican-hat or

champagne-bottle profile at the ground should benoted.

When the field Φ(x) is not coupled to electromagnetism, theMexican-hat potential has flat directions. Starting in any one of thecircle of vacua and changing the phase of the field from point to pointcosts very little energy. Mathematically, if

with a constant prefactor, then the action for the field θ(x), i.e., the "phase" of the Higgs field Φ(x), has onlyderivative terms. This is not a surprise. Adding a constant to θ(x) is a symmetry of the original theory, so differentvalues of θ(x) cannot have different energies. This is an example of Goldstone's theorem: spontaneously brokencontinuous symmetries normally produce massless excitations.The Abelian Higgs model is the Mexican-hat model coupled to electromagnetism:

The classical vacuum is again at the minimum of the potential, where the magnitude of the complex field φ is equalto Φ. But now the phase of the field is arbitrary, because gauge transformations change it. This means that the fieldθ(x) can be set to zero by a gauge transformation, and does not represent any actual degrees of freedom at all.Furthermore, choosing a gauge where the phase of the vacuum is fixed, the potential energy for fluctuations of thevector field is nonzero. So in the abelian Higgs model, the gauge field acquires a mass. To calculate the magnitude ofthe mass, consider a constant value of the vector potential A in the x direction in the gauge where the condensate hasconstant phase. This is the same as a sinusoidally varying condensate in the gauge where the vector potential is zero.In the gauge where A is zero, the potential energy density in the condensate is the scalar gradient energy:

And this energy is the same as a mass term ½m2A2 where m = qΦ.

Nonabelian Higgs mechanismThe Nonabelian Higgs model has the following action:

where now the nonabelian field A is contained in D and in the tensor components and (the relationbetween A and those components is well-known from the Yang–Mills theory).It is exactly analogous to the Abelian Higgs model. Now the field φ is in a representation of the gauge group, and thegauge covariant derivative is defined by the rate of change of the field minus the rate of change from parallel

Higgs mechanism 49

transport using the gauge field A as a connection.

Again, the expectation value of Φ defines a preferred gauge where the vacuum is constant, and fixing this gauge,fluctuations in the gauge field A come with a nonzero energy cost.Depending on the representation of the scalar field, not every gauge field acquires a mass. A simple example is in therenormalizable version of an early electroweak model due to Julian Schwinger. In this model, the gauge group isSO(3) (or SU(2)--- there are no spinor representations in the model), and the gauge invariance is broken down toU(1) or SO(2) at long distances. To make a consistent renormalizable version using the Higgs mechanism, introducea scalar field φa which transforms as a vector (a triplet) of SO(3). If this field has a vacuum expectation value, itpoints in some direction in field space. Without loss of generality, one can choose the z-axis in field space to be thedirection that φ is pointing, and then the vacuum expectation value of φ is (0, 0, A), where A is a constant withdimensions of mass ( ).Rotations around the z-axis form a U(1) subgroup of SO(3) which preserves the vacuum expectation value of φ, andthis is the unbroken gauge group. Rotations around the x and y-axis do not preserve the vacuum, and the componentsof the SO(3) gauge field which generate these rotations become massive vector mesons. There are two massive Wmesons in the Schwinger model, with a mass set by the mass scale A, and one massless U(1) gauge boson, similar tothe photon.The Schwinger model predicts magnetic monopoles at the electroweak unification scale, and does not predict the Zmeson. It doesn't break electroweak symmetry properly as in nature. But historically, a model similar to this (but notusing the Higgs mechanism) was the first in which the weak force and the electromagnetic force were unified.

Affine Higgs mechanismErnst Stueckelberg discovered a version of the Higgs mechanism by analyzing the theory of quantumelectrodynamics with a massive photon. Stueckelberg's model is a limit of the regular Mexican hat Abelian Higgsmodel, where the vacuum expectation value H goes to infinity and the charge of the Higgs field goes to zero in sucha way that their product stays fixed. The mass of the Higgs boson is proportional to H, so the Higgs boson becomesinfinitely massive and disappears. The vector meson mass is equal to the product eH, and stays finite.The interpretation is that when a U(1) gauge field does not require quantized charges, it is possible to keep only theangular part of the Higgs oscillations, and discard the radial part. The angular part of the Higgs field θ has thefollowing gauge transformation law:

The gauge covariant derivative for the angle (which is actually gauge invariant) is:

In order to keep θ fluctuations finite and nonzero in this limit, θ should be rescaled by H, so that its kinetic term inthe action stays normalized. The action for the theta field is read off from the Mexican hat action by substituting

.

since eH is the gauge boson mass. By making a gauge transformation to set θ = 0, the gauge freedom in the action iseliminated, and the action becomes that of a massive vector field:

Higgs mechanism 50

To have arbitrarily small charges requires that the U(1) is not the circle of unit complex numbers undermultiplication, but the real numbers R under addition, which is only different in the global topology. Such a U(1)group is non-compact. The field θ transforms as an affine representation of the gauge group. Among the allowedgauge groups, only non-compact U(1) admits affine representations, and the U(1) of electromagnetism isexperimentally known to be compact, since charge quantization holds to extremely high accuracy.The Higgs condensate in this model has infinitesimal charge, so interactions with the Higgs boson do not violatecharge conservation. The theory of quantum electrodynamics with a massive photon is still a renormalizable theory,one in which electric charge is still conserved, but magnetic monopoles are not allowed. For nonabelian gaugetheory, there is no affine limit, and the Higgs oscillations cannot be too much more massive than the vectors.

References[1] G. Bernardi, M. Carena, and T. Junk: "Higgs bosons: theory and searches", Reviews of Particle Data Group: Hypothetical particles and

Concepts, 2007, http:/ / pdg. lbl. gov/ 2008/ reviews/ higgs_s055. pdf[2] Guralnik, G S; Hagen, C R and Kibble, T W B (1967). Broken Symmetries and the Goldstone Theorem. Advances in Physics, vol. 2 (http:/ /

www. datafilehost. com/ download-7d512618. html)[3] Englert-Brout-Higgs-Guralnik-Hagen-Kibble Mechanism on Scholarpedia (http:/ / www. scholarpedia. org/ article/

Englert-Brout-Higgs-Guralnik-Hagen-Kibble_mechanism)[4] P. W. Anderson (1962). "Plasmons, Gauge Invariance, and Mass". Physical Review 130 (1): 439–442. Bibcode 1963PhRv..130..439A.

doi:10.1103/PhysRev.130.439.[5] Peter W. Higgs (1964). "Broken Symmetries and the Masses of Gauge Bosons". Physical Review Letters 13 (16): 508–509.

Bibcode 1964PhRvL..13..508H. doi:10.1103/PhysRevLett.13.508.[6] F. Englert and R. Brout (1964). "Broken Symmetry and the Mass of Gauge Vector Mesons". Physical Review Letters 13 (9): 321–323.

Bibcode 1964PhRvL..13..321E. doi:10.1103/PhysRevLett.13.321.[7] G. S. Guralnik, C. R. Hagen, and T. W. B. Kibble (1964). "Global Conservation Laws and Massless Particles". Physical Review Letters 13

(20): 585–587. Bibcode 1964PhRvL..13..585G. doi:10.1103/PhysRevLett.13.585.[8] Gerald S. Guralnik (2009). "The History of the Guralnik, Hagen and Kibble development of the Theory of Spontaneous Symmetry Breaking

and Gauge Particles". International Journal of Modern Physics A24 (14): 2601–2627. arXiv:0907.3466. Bibcode 2009IJMPA..24.2601G.doi:10.1142/S0217751X09045431.

[9] Nambu, Y (1960). "Quasiparticles and Gauge Invariance in the Theory of Superconductivity". Physical Review 117 (3): 648–663.Bibcode 1960PhRv..117..648N. doi:10.1103/PhysRev.117.648.

[10] Higgs, Peter (2007). "Prehistory of the Higgs boson". Comptes Rendus Physique 8 (9): 970–972. Bibcode 2007CRPhy...8..970H.doi:10.1016/j.crhy.2006.12.006

[11] Physical Review Letters – 50th Anniversary Milestone Papers (http:/ / prl. aps. org/ 50years/ milestones#1964)[12] American Physical Society – J. J. Sakurai Prize Winners (http:/ / www. aps. org/ units/ dpf/ awards/ sakurai. cfm)[13] Rochester's Hagen Sakurai Prize Announcement (http:/ / www. pas. rochester. edu/ urpas/ news/ Hagen_030708)[14] C.R. Hagen discusses naming of Higgs Boson in 2010 Sakurai Prize Talk (http:/ / www. youtube. com/ watch?v=QrCPrwRBi7E&

feature=PlayList& p=BDA16F52CA3C9B1D& playnext_from=PL& index=9)[15] Anything but the God particle by Ian Sample (http:/ / www. guardian. co. uk/ science/ blog/ 2009/ may/ 29/

why-call-it-the-god-particle-higgs-boson-cern-lhc)[16] G. 't Hooft and M. Veltman (1972). "Regularization and Renormalization of Gauge Fields". Nuclear Physics B 44 (1): 189–219.

Bibcode 1972NuPhB..44..189T. doi:10.1016/0550-3213(72)90279-9.[17] Regularization and Renormalization of Gauge Fields by t'Hooft and Veltman (PDF) (http:/ / igitur-archive. library. uu. nl/ phys/

2005-0622-155148/ 13877. pdf)

Higgs mechanism 51

Further reading• Schumm, Bruce A. (2004) Deep Down Things. Johns Hopkins Univ. Press. Chpt. 9.

External links• Guralnik, G.S.; Hagen, C.R.; Kibble, T.W.B. (1964). "Global Conservation Laws and Massless Particles".

Physical Review Letters 13 (20): 585–87. Bibcode 1964PhRvL..13..585G. doi:10.1103/PhysRevLett.13.585.• Mark D. Roberts (1999) " A Generalized Higgs Model. (http:/ / www. arXiv. org/ abs/ hep-th/ 9904080)"• Sakurai Prize Videos (http:/ / www. youtube. com/ view_play_list?p=BDA16F52CA3C9B1D)• In CERN Courier, Steven Weinberg reflects on spontaneous symmetry breaking (http:/ / cerncourier. com/ cws/

article/ cern/ 32522)• Steven Weinberg on LHC (http:/ / www. youtube. com/ watch?v=Zl4W3DYTIKw)• Steven Weinberg Praises Teams for Higgs Boson Theory. (http:/ / www. pas. rochester. edu/ urpas/ news/

Hagen_030708)• Physical Review Letters – 50th Anniversary Milestone Papers. (http:/ / prl. aps. org/ 50years/ milestones#1964)• Imperial College London on PRL 50th Anniversary Milestone Papers. (http:/ / www3. imperial. ac. uk/

newsandeventspggrp/ imperialcollege/ newssummary/ news_13-6-2008-12-42-20?newsid=38514)• " Introducing the little Higgs. (http:/ / physicsworld. com/ cws/ article/ print/ 11353)" Physics World.• Englert-Brout-Higgs-Guralnik-Hagen-Kibble Mechanism on Scholarpedia. (http:/ / www. scholarpedia. org/

article/ Englert-Brout-Higgs-Guralnik-Hagen-Kibble_mechanism)• History of Englert-Brout-Higgs-Guralnik-Hagen-Kibble Mechanism on Scholarpedia. (http:/ / www.

scholarpedia. org/ article/ Englert-Brout-Higgs-Guralnik-Hagen-Kibble_mechanism_(history))• The Hunt for the Higgs at Tevatron (http:/ / apps3. aps. org/ aps/ meetings/ april10/ roser. pdf)• The Mystery of Empty Space (https:/ / www. youtube. com/ watch?v=Y-vKh_jKX7Q) on YouTube. A lecture

with UCSD physicist Kim Griest (43 minutes).

Large Hadron Collider 52

Large Hadron Collider

Large Hadron Collider(LHC)

LHC experiments

ATLAS A Toroidal LHC Apparatus

CMS Compact Muon Solenoid

LHCb LHC-beauty

ALICE A Large Ion Collider Experiment

TOTEM Total Cross Section, Elastic Scattering and Diffraction Dissociation

LHCf LHC-forward

MoEDAL Monopole and Exotics Detector At the LHC

LHC preaccelerators

p and Pb Linear accelerators for protons (Linac 2) and Lead (Linac 3)

(not marked) Proton Synchrotron Booster

PS Proton Synchrotron

SPS Super Proton Synchrotron

Hadron colliders

Intersecting Storage Rings CERN, 1971–1984

Super Proton Synchrotron CERN, 1981–1984

ISABELLE BNL, cancelled in 1983

Tevatron Fermilab, 1987–2011

Relativistic Heavy Ion Collider BNL, 2000–present

Superconducting Super Collider Cancelled in 1993

Large Hadron Collider CERN, 2009–present

Super Large Hadron Collider Proposed, CERN, 2019–

Very Large Hadron Collider Theoretical

The Large Hadron Collider (LHC) is the world's largest and highest-energy particle accelerator. It is expected toaddress some of the most fundamental questions of physics, advancing the understanding of the deepest laws ofnature.

Large Hadron Collider 53

The LHC lies in a tunnel 27 kilometres (17 mi) in circumference, as deep as 175 metres (574 ft) beneath theFranco-Swiss border near Geneva, Switzerland. This synchrotron is designed to collide opposing particle beams ofeither protons at an energy of 7 teraelectronvolts (7 TeV or 1.12 microjoules) per nucleon, or lead nuclei at an energyof 574 TeV (92.0 µJ) per nucleus (2.76 TeV per nucleon).[1] [2] The term hadron refers to particles composed ofquarks.The Large Hadron Collider was built by the European Organization for Nuclear Research (CERN) with the intentionof testing various predictions of high-energy physics, including testing for the existence of the hypothesized Higgsboson[3] and of the large family of new particles predicted by supersymmetry.[4] It was built in collaboration withover 10,000 scientists and engineers from over 100 countries, as well as hundreds of universities and laboratories.[5]

On 10 September 2008, the proton beams were successfully circulated in the main ring of the LHC for the firsttime,[6] but 9 days later operations were halted due to an explosion involving helium gas.[7] [8] On 20 November2009 they were successfully circulated again,[9] with the first recorded proton–proton collisions occurring 3 dayslater at the injection energy of 450 GeV per beam.[10] After the 2009 winter shutdown, the LHC was restarted andthe beam was ramped up to 3.5 TeV per beam[11] (half its designed energy).[12] On 30 March 2010, the first plannedcollisions took place between two 3.5 TeV beams, a new world record for the highest-energy man-made particlecollisions.[13] The LHC will continue to operate at half energy until the end of 2012; it will not run at full energy (7TeV per beam) until 2014.[14]

Purpose

A simulated event in the CMS detector, featuringthe appearance of the Higgs boson

Physicists hope that the LHC will help answer some of thefundamental open questions in physics, concerning the basic lawsgoverning the interactions and forces among the elementary objects,the deep structure of space and time, and in particular the intersectionof quantum mechanics and general relativity, where current theoriesand knowledge are unclear or break down altogether. Data is alsoneeded from high energy particle experiments to indicate whichversions of scientific models are more likely to be correct - inparticular to choose between the Standard Model and Higgsless modelsand to validate their predictions and allow further theoreticaldevelopment. Many theorists expect new physics beyond the StandardModel to emerge at the TeV-scale, based on unsatisfactory propertiesof the Standard Model. Issues possibly to be explored by LHCcollisions include:[15]

• Is the Higgs mechanism for generating elementary particle masses via electroweak symmetry breaking actuallyrealised in nature?[16] It is expected that the collider will either demonstrate or rule out the existence of theelusive Higgs boson, thereby allowing physicists to determine whether the Standard Model or its Higgslessmodel alternatives are more likely to be correct.[17] [18] [19]

• Is supersymmetry, an extension of the Standard Model and Poincaré symmetry, realised in nature, implyingthat all known particles have supersymmetric partners?[20] [21] [22]

• Are there extra dimensions,[23] as predicted by various models based on string theory, and can we detectthem?[24]

• What is the nature of the dark matter that appears to account for 23% of the mass of the universe?Other open questions which may be explored using high energy particle collisions:

• It is already known that electromagnetism and the weak nuclear force are just different manifestations of a single force called the electroweak force. The LHC may clarify whether the electroweak force and the strong

Large Hadron Collider 54

nuclear force are similarly just different manifestations of one universal unified force, as predicted by variousGrand Unification Theories.

• Why is the fourth fundamental force (gravity) so many orders of magnitude weaker than the other threefundamental forces? See also Hierarchy problem.

• Are there additional sources of quark flavour mixing, beyond those already predicted within the StandardModel?

• Why are there apparent violations of the symmetry between matter and antimatter? See also CP violation.• What are the nature and properties of quark-gluon plasma, believed to have existed in the early universe and in

certain compact and strange astronomical objects today? This will be investigated by heavy ion collisions inALICE.

Design

A Feynman diagram of one way the Higgs bosonmay be produced at the LHC. Here, two quarkseach emit a W or Z boson, which combine to

make a neutral Higgs.

Map of the Large Hadron Collider at CERN

The LHC is the world's largest and highest-energy particleaccelerator.[1] [25] The collider is contained in a circular tunnel, with acircumference of 27 kilometres (17 mi), at a depth ranging from 50 to175 metres (160 to 574 ft) underground.

The 3.8-metre (12 ft) wide concrete-lined tunnel, constructed between1983 and 1988, was formerly used to house the LargeElectron–Positron Collider.[26] It crosses the border betweenSwitzerland and France at four points, with most of it in France.Surface buildings hold ancillary equipment such as compressors,ventilation equipment, control electronics and refrigeration plants.

The collider tunnel contains two adjacent parallel beam pipes thatintersect at four points, each containing a proton beam, which travel inopposite directions around the ring. Some 1,232 dipole magnets keepthe beams on their circular path, while an additional 392 quadrupolemagnets are used to keep the beams focused, in order to maximize thechances of interaction between the particles in the four intersectionpoints, where the two beams will cross. In total, over 1,600superconducting magnets are installed, with most weighing over 27tonnes. Approximately 96 tonnes of liquid helium is needed to keep themagnets, made of copper-clad niobium-titanium, at their operatingtemperature of 1.9 K (−271.25 °C), making the LHC the largestcryogenic facility in the world at liquid helium temperature.

Large Hadron Collider 55

Superconducting quadrupole electromagnets areused to direct the beams to four intersection

points, where interactions between acceleratedprotons will take place.

Once or twice a day, as the protons are accelerated from 450 GeV to7 TeV, the field of the superconducting dipole magnets will beincreased from 0.54 to 8.3 teslas (T). The protons will each have anenergy of 7 TeV, giving a total collision energy of 14 TeV. At thisenergy the protons have a Lorentz factor of about 7,500 and move atabout 0.999999991 c, or about 3 metres per second slower than thespeed of light (c).[27] It will take less than 90 microseconds (μs) for aproton to travel once around the main ring – a speed of about 11,000revolutions per second. Rather than continuous beams, the protons willbe bunched together, into 2,808 bunches, so that interactions betweenthe two beams will take place at discrete intervals never shorter than 25nanoseconds (ns) apart. However it will be operated with fewerbunches when it is first commissioned, giving it a bunch crossing

interval of 75 ns.[28] The design luminosity of the LHC is 1034 cm−2s−1, providing a bunch collision rate of40 MHz.[29]

Prior to being injected into the main accelerator, the particles are prepared by a series of systems that successivelyincrease their energy. The first system is the linear particle accelerator LINAC 2 generating 50-MeV protons, whichfeeds the Proton Synchrotron Booster (PSB). There the protons are accelerated to 1.4 GeV and injected into theProton Synchrotron (PS), where they are accelerated to 26 GeV. Finally the Super Proton Synchrotron (SPS) is usedto further increase their energy to 450 GeV before they are at last injected (over a period of 20 minutes) into themain ring. Here the proton bunches are accumulated, accelerated (over a period of 20 minutes) to their peak 7-TeVenergy, and finally circulated for 10 to 24 hours while collisions occur at the four intersection points.[30]

CMS detector for LHC

The LHC physics program is mainly based on proton–proton collisions.However, shorter running periods, typically one month per year, with heavy-ioncollisions are included in the program. While lighter ions are considered as well,the baseline scheme deals with lead ions[31] (see A Large Ion ColliderExperiment). The lead ions will be first accelerated by the linear acceleratorLINAC 3, and the Low-Energy Ion Ring (LEIR) will be used as an ion storageand cooler unit. The ions will then be further accelerated by the PS and SPSbefore being injected into LHC ring, where they will reach an energy of 2.76TeV per nucleon (or 575 TeV per ion), higher than the energies reached by theRelativistic Heavy Ion Collider. The aim of the heavy-ion program is toinvestigate quark–gluon plasma, which existed in the early universe.

Detectors

Six detectors have been constructed at the LHC, located underground in large caverns excavated at the LHC'sintersection points. Two of them, the ATLAS experiment and the Compact Muon Solenoid (CMS), are large, generalpurpose particle detectors.[25] A Large Ion Collider Experiment (ALICE) and LHCb, have more specific roles andthe last two, TOTEM and LHCf, are very much smaller and are for very specialized research. The BBC's summaryof the main detectors is:[32]

Large Hadron Collider 56

Detector Description

ATLAS one of two general purpose detectors. ATLAS will be used to look for signs of new physics, including the origins of mass and extradimensions.

CMS the other general purpose detector will, like ATLAS, hunt for the Higgs boson and look for clues to the nature of dark matter.

ALICE is studying a "fluid" form of matter called quark–gluon plasma that existed shortly after the Big Bang.

LHCb equal amounts of matter and antimatter were created in the Big Bang. LHCb will try to investigate what happened to the "missing"antimatter.

Operational historyThe first beam was circulated through the collider on the morning of 10 September 2008.[32] CERN successfullyfired the protons around the tunnel in stages, three kilometres at a time. The particles were fired in a clockwisedirection into the accelerator and successfully steered around it at 10:28 local time.[33] The LHC successfullycompleted its first major test: after a series of trial runs, two white dots flashed on a computer screen showing theprotons travelled the full length of the collider. It took less than one hour to guide the stream of particles around itsinaugural circuit.[34] CERN next successfully sent a beam of protons in a counterclockwise direction, taking slightlylonger at one and a half hours due to a problem with the cryogenics, with the full circuit being completed at 14:59.On 19 September 2008, a quench occurred in about 100 bending magnets in sectors 3 and 4, causing a loss ofapproximately six tonnes of liquid helium, which was vented into the tunnel, and a temperature rise of about 100kelvin in some of the affected magnets. Vacuum conditions in the beam pipe were also lost.[35] Shortly after theincident CERN reported that the most likely cause of the problem was a faulty electrical connection between twomagnets, and that – due to the time needed to warm up the affected sectors and then cool them back down tooperating temperature – it would take at least two months to fix it.[36] Subsequently, CERN released a preliminaryanalysis of the incident on 16 October 2008,[37] and a more detailed one on 5 December 2008.[38] Both analysesconfirmed that the incident was indeed initiated by a faulty electrical connection. A total of 53 magnets weredamaged in the incident and were repaired or replaced during the winter shutdown.[39]

In the original timeline of the LHC commissioning, the first "modest" high-energy collisions at a center-of-massenergy of 900 GeV were expected to take place before the end of September 2008, and the LHC was expected to beoperating at 10 TeV by the time of the official inauguration on 21 October 2008.[40] However, due to the delaycaused by the above-mentioned incident, the collider was not operational until November 2009.[41] Despite thedelay, LHC was officially inaugurated on 21 October 2008, in the presence of political leaders, science ministersfrom CERN's 20 Member States, CERN officials, and members of the worldwide scientific community.[42]

Most of 2009 was spent on repairs and reviews from the damage caused by the quench incident. On 20 November,low-energy beams circulated in the tunnel for the first time since the incident. The early part of 2010 saw thecontinued ramp-up of beam in energies and early physics experiments. On 30 March 2010, LHC set a record forhigh-energy collisions, by colliding proton beams at a combined energy level of 7 TeV. The attempt was the thirdthat day, after two unsuccessful attempts in which the protons had to be "dumped" from the collider and new beamshad to be injected.[43] The first proton run ended on 4 November 2010. A run with lead ions started on 8 November2010, and ended on 6 December 2010.[44] This allowed the ALICE experiment to study matter under extremeconditions similar to those shortly after the Big Bang.[45]

CERN has declared that the LHC will run through to the end of 2012, with a short technical stop at the end of 2011.The energy for 2011 will be 3.5 TeV per beam. In 2013 the LHC will go into a long shutdown to prepare forhigher-energy running starting in 2014.[14]

Large Hadron Collider 57

Timeline

Date Event

10 Sep 2008 CERN successfully fired the first protons around the entire tunnel circuit in stages.

19 Sep 2008 Magnetic quench occurred in about 100 bending magnets in sectors 3 and 4, causing a loss of approximately 6 tonnes of liquidhelium.

30 Sep 2008 First "modest" high-energy collisions planned but postponed due to accident.

16 Oct 2008 CERN released a preliminary analysis of the accident.

21 Oct 2008 Official inauguration.

5 Dec 2008 CERN released detailed analysis.

20 Nov 2009 Low-energy beams circulated in the tunnel for the first time since the accident.[46]

23 Nov 2009 First particle collisions in all four detectors at 450 GeV.[10]

30 Nov 2009 LHC becomes the world's highest-energy particle accelerator achieving 1.18 TeV per beam, beating the Tevatron's previous record of0.98 TeV per beam held for eight years.[47]

28 Feb 2010 The LHC continues operations ramping energies to run at 3.5 TeV for 18 months to two years, after which it will be shut down toprepare for the 14 TeV collisions (7 TeV per beam).[48]

30 Mar 2010 The two beams collided at 7 TeV (3.5 TeV per beam) in the LHC at 13:06 CEST, marking the start of the LHC research program.

8 Nov 2010 Start of the first run with lead ions.

6 Dec 2010 End of the run with lead ions. Shutdown until early 2011.

13 Mar 2011 Beginning of the 2011 run with proton beams.[49]

21 Apr 2011 LHC becomes the world's highest-luminosity hadron accelerator achieving a peak luminosity of 4.67·1032 cm−2s−1, beating theTevatron's previous record of 4·1032 cm−2s−1 held for one year.[50]

17 Jun 2011 The high luminosity experiments ATLAS and CMS reach 1 fb-1 of collected data.[51]

23 Oct 2011 The high luminosity experiments ATLAS and CMS reach 5 fb-1 of collected data.

FindingsCERN scientists estimate that, if the Standard Model is correct, a single Higgs boson may be produced every fewhours. At this rate, it may take about two to three years to collect enough data to discover the Higgs bosonunambiguously. Similarly, it may take one year or more before sufficient results concerning supersymmetricparticles have been gathered to draw meaningful conclusions.[1] On the other hand, some extensions of the StandardModel predict additional particles, such as the heavy W' and Z' gauge bosons, whose existence might already beprobed after a few months of data collection.[52]

The first physics results from the LHC, involving 284 collisions which took place in the ALICE detector, werereported on 15 December 2009.[53] The results of the first proton–proton collisions at energies higher than Fermilab'sTevatron proton–antiproton collisions were published by the CMS collaboration in early February 2010, yieldinggreater-than-predicted charged-hadron production.[54]

After the first year of data collection, the LHC experimental collaborations started to release their preliminary resultsconcerning searches for new physics beyond the Standard Model in proton-proton collisions.[55] [56] [57] [58] Noevidence of new particles was detected in the 2010 data. As a result, bounds were set on the allowed parameter spaceof various extensions of the Standard Model, such as models with large extra dimensions, constrained versions of theMinimal Supersymmetric Standard Model, and others.[59] [60] [61]

Large Hadron Collider 58

On 24 May 2011 it was reported that quark–gluon plasma (the densest matter besides black holes) has been createdin the LHC.[62]

Between July and August 2011, results of searches for the Higgs boson and for exotic particles, based on the datacollected during the first half of the 2011 run, were presented in conferences in Grenoble[63] and Mumbai.[64] In thelatter conference it was reported that, despite hints of a Higgs signal in earlier data, ATLAS and CMS exclude with95% confidence level the existence of a Higgs boson with the properties predicted by the Standard Model over mostof the mass region between 145 and 466 GeV.[65] The searches for new particles did not yield signals either,allowing to further constrain the parameter space of various extensions of the Standard Model, including itssupersymmetric extensions.[66] [67]

Proposed upgradeAfter some years of running, any particle physics experiment typically begins to suffer from diminishing returns:each additional year of operation discovers less than the year before. The way around the diminishing returns is toupgrade the experiment, either in energy or in luminosity. A luminosity upgrade of the LHC, called the Super LHC,has been proposed,[68] to be made in 2018 after ten years of operation.The optimal path for the LHC luminosity upgrade includes an increase in the beam current (i.e. the number ofprotons in the beams) and the modification of the two high-luminosity interaction regions, ATLAS and CMS. Toachieve these increases, the energy of the beams at the point that they are injected into the (Super) LHC should alsobe increased to 1 TeV. This will require an upgrade of the full pre-injector system, the needed changes in the SuperProton Synchrotron being the most expensive. Currently the collaborative research effort of LHC AcceleratorResearch Program, LARP is conducting research into how to achieve these goals.[69]

CostWith a budget of 7.5 billion euros (approx. $9bn or £6.19bn as of Jun 2010), the LHC is one of the most expensivescientific instruments[70] ever built.[71] The total cost of the project is expected to be of the order of 4.6bn Swissfrancs (approx. $4.4bn, €3.1bn, or £2.8bn as of Jan 2010) for the accelerator and SFr 1.16bn (approx. $1.1bn,€0.8bn, or £0.7bn as of Jan 2010) for the CERN contribution to the experiments.[72]

The construction of LHC was approved in 1995 with a budget of SFr 2.6bn, with another SFr 210M towards theexperiments. However, cost overruns, estimated in a major review in 2001 at around SFr 480M for the accelerator,and SFr 50M for the experiments, along with a reduction in CERN's budget, pushed the completion date from 2005to April 2007.[73] The superconducting magnets were responsible for SFr 180M of the cost increase. There were alsofurther costs and delays due to engineering difficulties encountered while building the underground cavern for theCompact Muon Solenoid,[74] and also due to faulty parts provided by Fermilab.[75] Due to lower electricity costsduring the summer, it is expected that the LHC will normally not operate over the winter months,[76] although anexception was made to make up for the 2008 start-up delays over the 2009/10 winter.

Computing resourcesData produced by LHC, as well as LHC-related simulation, was estimated at approximately 15 petabytes per year(max throughput while running not stated).[77]

The LHC Computing Grid[78] was constructed to handle the massive amounts of data produced. It incorporated bothprivate fiber optic cable links and existing high-speed portions of the public Internet, enabling data transfer fromCERN to academic institutions around the world.[79]

The Open Science Grid is used as the primary infrastructure in the United States, and also as part of an interoperablefederation with the LHC Computing Grid.

Large Hadron Collider 59

The distributed computing project LHC@home was started to support the construction and calibration of the LHC.The project uses the BOINC platform, enabling anybody with an Internet connection and either Windows, Mac OSX or Linux to use their computer's idle time to simulate how particles will travel in the tunnel. With this information,the scientists will be able to determine how the magnets should be calibrated to gain the most stable "orbit" of thebeams in the ring.[80]

Safety of particle collisionsThe experiments at the Large Hadron Collider sparked fears among the public that the particle collisions mightproduce doomsday phenomena, involving the production of stable microscopic black holes or the creation ofhypothetical particles called strangelets.[81] Two CERN-commissioned safety reviews examined these concerns andconcluded that the experiments at the LHC present no danger and that there is no reason for concern,[82] [83] [84] aconclusion expressly endorsed by the American Physical Society.[85]

Operational challengesThe size of the LHC constitutes an exceptional engineering challenge with unique operational issues on account ofthe amount of energy stored in the magnets and the beams.[30] [86] While operating, the total energy stored in themagnets is 10 GJ (equivalent to 2.4 tons of TNT) and the total energy carried by the two beams reaches 724 MJ(173 kilograms of TNT).[87]

Loss of only one ten-millionth part (10−7) of the beam is sufficient to quench a superconducting magnet, while thebeam dump must absorb 362 MJ (87 kilograms of TNT) for each of the two beams. These energies are carried byvery little matter: under nominal operating conditions (2,808 bunches per beam, 1.15×1011 protons per bunch), thebeam pipes contain 1.0×10−9 gram of hydrogen, which, in standard conditions for temperature and pressure, wouldfill the volume of one grain of fine sand.

Construction accidents and delays• On 25 October 2005, José Pereira Lages, a technician, was killed in the LHC when a switchgear that was being

transported fell on him.[88]

• On 27 March 2007 a cryogenic magnet support broke during a pressure test involving one of the LHC's innertriplet (focusing quadrupole) magnet assemblies, provided by Fermilab and KEK. No one was injured. Fermilabdirector Pier Oddone stated "In this case we are dumbfounded that we missed some very simple balance offorces". This fault had been present in the original design, and remained during four engineering reviews over thefollowing years.[89] Analysis revealed that its design, made as thin as possible for better insulation, was not strongenough to withstand the forces generated during pressure testing. Details are available in a statement fromFermilab, with which CERN is in agreement.[90] [91] Repairing the broken magnet and reinforcing the eightidentical assemblies used by LHC delayed the startup date, then planned for November 2007.

• Problems occurred on 19 September 2008 during powering tests of the main dipole circuit, when an electricalfault in the bus between magnets caused a rupture and a leak of six tonnes of liquid helium. The operation wasdelayed for several months.[92] It is currently believed that a faulty electrical connection between two magnetscaused an arc, which compromised the liquid-helium containment. Once the cooling layer was broken, the heliumflooded the surrounding vacuum layer with sufficient force to break 10-ton magnets from their mountings. Theexplosion also contaminated the proton tubes with soot.[38] [93] This accident was thoroughly discussed in a22 February 2010 Superconductor Science and Technology article by CERN physicist Lucio Rossi.[94]

• Two vacuum leaks were identified in July 2009, and the start of operations was further postponed tomid-November 2009.[95]

Large Hadron Collider 60

Popular cultureThe Large Hadron Collider gained a considerable amount of attention from outside the scientific community and itsprogress is followed by most popular science media. The LHC has also sparked the imaginations of authors of worksof fiction, such as novels, TV series, and video games, although descriptions of what it is, how it works, andprojected outcomes of the experiments are often only vaguely accurate, occasionally causing concern among thegeneral public.The novel Angels & Demons, by Dan Brown, involves antimatter created at the LHC to be used in a weapon againstthe Vatican. In response CERN published a "Fact or Fiction?" page discussing the accuracy of the book's portrayal ofthe LHC, CERN, and particle physics in general.[96] The movie version of the book has footage filmed on-site at oneof the experiments at the LHC; the director, Ron Howard, met with CERN experts in an effort to make the science inthe story more accurate.[97]

In The Big Bang Theory, the episode "The Large Hadron Collision" features the Large Hadron Collider prominently- albeit as a concept (the collider itself is not shown).The novel FlashForward, by Robert J. Sawyer, involves the search for the Higgs boson at the LHC. CERNpublished a "Science and Fiction" page interviewing Sawyer and physicists about the book and the TV series basedon it.[98]

CERN employee Katherine McAlpine's "Large Hadron Rap"[99] surpassed 6.7 million YouTube views.[100] [101] Theband Les Horribles Cernettes was founded by female members of CERN. The name was chosen so to have the sameinitials as the LHC.[102] [103]

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External links• Official website (http:/ / lhc. web. cern. ch/ lhc/ )• Overview of the LHC at CERN's public webpage (http:/ / public. web. cern. ch/ public/ en/ LHC/ LHC-en. html)• CERN Courier magazine (http:/ / www. cerncourier. com/ )• CERN (https:/ / twitter. com/ cern) on Twitter• CMS Experiment at CERN (https:/ / twitter. com/ CMSExperiment) on Twitter• Unofficial CERN (https:/ / twitter. com/ LHCExperiment) on Twitter• LHC Portal (http:/ / www. lhcportal. com/ ) Web portal• Lyndon Evans and Philip Bryant (eds) (2008). "LHC Machine" (http:/ / www. iop. org/ EJ/ journal/ -page=extra.

lhc/ jinst). Journal of Instrumentation 3 (8): S08001. Bibcode 2008JInst...3S8001E.doi:10.1088/1748-0221/3/08/S08001. Full documentation for design and construction of the LHC and its sixdetectors (1600p).

• symmetry magazine LHC special issue August 2006 (http:/ / www. symmetrymagazine. org/ cms/ ?pid=1000350),special issue December 2007 (http:/ / www. symmetrymagazine. org/ cms/ ?pid=1000562)

• New Yorker: Crash Course (http:/ / www. newyorker. com/ reporting/ 2007/ 05/ 14/ 070514fa_fact_kolbert). Theworld's largest particle accelerator.

• NYTimes: A Giant Takes On Physics' Biggest Questions (http:/ / www. nytimes. com/ 2007/ 05/ 15/ science/15cern. html?ex=1336881600& en=7825f6702d7071e7& ei=5090& partner=rssuserland& emc=rss).

• Why a Large Hadron Collider? (http:/ / seedmagazine. com/ news/ 2006/ 07/ why_a_large_hadron_collider. php)Seed Magazine interviews with physicists.

• Thirty collected pictures during commissioning and post- 19 September 2008 incident repair (http:/ / www.boston. com/ bigpicture/ 2009/ 11/ large_hadron_collider_ready_to. html), from Boston Globe.

• Podcast Interview (http:/ / omegataupodcast. net/ 2010/ 03/ 30-the-large-hadron-collider/ ) with CERN's RolfLandua about the LHC and the physics behind it

Peter Higgs 65

Peter Higgs

Peter Higgs

Born 29 May 1929Newcastle upon Tyne, England

Nationality British

Fields Physics

Institutions University of EdinburghImperial College LondonUniversity College London

Alma mater King's College London

Doctoral advisor Charles Coulson

Doctoral students Christopher BishopLewis RyderDavid Wallace

Known for Broken symmetry in electroweak theory

Notable awards Wolf Prize in Physics (2004)Sakurai Prize (2010)Dirac Medal

Peter Ware Higgs, FRS, FRSE, FKC (born 29 May 1929), is an English theoretical physicist and an emeritusprofessor at the University of Edinburgh.[1]

He is best known for his 1960s proposal of broken symmetry in electroweak theory, explaining the origin of mass ofelementary particles in general and of the W and Z bosons in particular. This so-called Higgs mechanism, which hadseveral inventors besides Higgs at about the same time, predicts the existence of a new particle, the Higgs boson(often described as "the most sought-after particle in modern physics"[2] [3] ). Although this particle has not turned upin accelerator experiments so far, the Higgs mechanism is generally accepted as an important ingredient in theStandard Model of particle physics, without which particles would have no mass.[4]

Prof. Higgs has been honored with a number of awards in recognition of his work, including the 1997 Dirac Medaland Prize for outstanding contributions to theoretical physics from the Institute of Physics, the 1997 High Energyand Particle Physics Prize by the European Physical Society, the 2004 Wolf Prize in Physics, and the 2010 J. J.Sakurai Prize for Theoretical Particle Physics.

Peter Higgs 66

Early life, education and careerHiggs was born in Wallsend, Newcastle upon Tyne.[5] His father worked as a sound engineer for the BBC, and as aresult of childhood asthma, together with the family moving around because of his father's job, and later the SecondWorld War, Higgs missed some early schooling and was taught at home. When his father relocated to Bedford,Higgs stayed behind with his mother in Bristol, and was largely raised there. He attended that city's CothamGrammar School,[6] where he was inspired by the work of one of the school's alumni, Paul Dirac, a founder of thefield of quantum mechanics.[5]

At the age of 17 Higgs moved to City of London School, where he specialized in mathematics, then to King'sCollege London where he graduated with a first class honours degree in Physics, a masters degree, and Ph.D.[1] Hebecame a Senior Research Fellow at the Edinburgh University, then held various posts at University College Londonand Imperial College London before becoming a temporary lecturer in Mathematics at University College London.He returned to Edinburgh University in 1960 to take up the post of Lecturer at the Tait Institute of MathematicalPhysics, allowing him to settle in the city he had fallen in love with after hitch-hiking to the Edinburgh Fringefestival as a student.[1]

Dr. Higgs was promoted to a personal chair of Theoretical Physics at Edinburgh in 1980. He became a fellow of theRoyal Society in 1983, was awarded the Rutherford Medal and Prize in 1984, and became a fellow of the Institute ofPhysics in 1991. He retired in 1996 and became Emeritus professor at the University of Edinburgh.[1]

Work in theoretical physicsAt Edinburgh Higgs first became interested in mass, developing the idea that particles were massless when theuniverse began, acquiring mass a fraction of a second later as a result of interacting with a theoretical field (whichbecame known as the Higgs field). Higgs postulated that this field permeates space, giving all elementary subatomicparticles that interact with it their mass.[5] [7] While the Higgs field is postulated to confer mass on quarks andleptons, it causes only a tiny portion of the masses of other subatomic particles, such as protons and neutrons. Inthese, gluons that bind quarks together confer most of the particle mass.The original basis of Higgs' work came from the Japanese-born theorist and Nobel Prize winner Yoichiro Nambufrom the University of Chicago. Professor Nambu had proposed a theory known as spontaneous symmetry breakingbased on what was known to happen in superconductivity in condensed matter. However, the theory predictedmassless particles (the Goldstone's theorem), a clearly incorrect prediction.[1]

Higgs wrote a short paper exploiting a loophole in Goldstone's theorem and published it in Physics Letters, aEuropean physics journal edited at CERN, in 1964.[8]

Higgs wrote a second paper describing a theoretical model (now called the Higgs mechanism) but the paper wasrejected (the editors of Physics Letters judged it "of no obvious relevance to physics"[5] ). Higgs wrote an extraparagraph and sent his paper to Physical Review Letters, another leading physics-journal, which published it laterthat year.[9] Other physicists, Robert Brout and Francois Englert[10] and Gerald Guralnik, C. R. Hagen, and TomKibble[11] had reached the same conclusion independently about the same time. The three papers written on thisboson discovery by Higgs, Guralnik, Hagen, Kibble, Brout, and Englert were each recognized as milestone papers byPhysical Review Letters 50th anniversary celebration.[12] While each of these famous papers took similarapproaches, the contributions and differences between the 1964 PRL symmetry breaking papers are noteworthy.Nobelist Philip Anderson also claims to have "invented" the "Higgs" boson as far back as 1962.As an atheist, Higgs is reported to be displeased that the particle is nicknamed the "God particle". Higgs is afraid theterm "might offend people who are religious".[13] [14] This nickname for the Higgs boson is usually attributed toLeon Lederman, but it is actually the result of Lederman's publisher's censoring. Originally Lederman intended tocall it "the goddamn particle", because of its elusiveness.[15]

Peter Higgs 67

The Large Hadron Collider at CERN in Switzerland detected possible signs of the Higgs boson in July 2011, afinding repeated by researchers at the Tevatron near Chicago. They reported "interesting particle events at a mass ofbetween 140 and 145 gigaelectronvolts".[16] If the Higgs boson is found at CERN (ironically, home to the editor whofamously rejected his initial paper), Higgs and the others who contributed to theBrout-Englert-Higgs-Guralnik-Hagen-Kibble mechanism may receive a Nobel Prize.[1]

Political viewsHiggs was a CND activist while in London and later in Edinburgh, but resigned his membership when the groupextended its remit from campaigning against nuclear weapons to campaigning against nuclear power too.[17] [5] Hewas a Greenpeace member until the group opposed genetically modified organisms.[17]

Higgs was awarded the 2004 Wolf Prize in Physics (sharing it with Brout and Englert), but he refused to fly toJerusalem to receive the award because it was a state occasion attended by the then President of Israel, MosheKatsav, and Higgs is opposed to Israel's actions in Palestine.[18]

FamilyHiggs has two sons: Chris, a computer scientist, and Jonny, a jazz musician.[13]

Cultural referencesA portrait of Peter Higgs was painted by Ken Currie in 2008.[19] Commissioned by the University of Edinburgh,[20]

(subscription required) it was unveiled on 3 April 2009[21] and hangs in the entrance of the James Clerk MaxwellBuilding of the School of Physics and Astronomy.[19]

Notes and references[1] Griggs, Jessica. "The Missing Piece" from Edit the University of Edinburgh Alumni Magazine Summer 2008, Page 17[2] Griffiths, Martin (20070501) physicsworld.com The Tale of the Blog's Boson (http:/ / physicsworld. com/ cws/ article/ print/ 27731)

Retrieved on 2008-05-27[3] Fermilab Today (20050616) Fermilab Results of the Week. Top Quarks are Higgs' best Friend (http:/ / www. fnal. gov/ pub/ today/

archive_2005/ today05-06-16. html) Retrieved on 2008-05-27[4] Rincon, Paul (20040310) Fermilab 'God Particle' may have been seen (http:/ / news. bbc. co. uk/ 2/ hi/ science/ nature/ 3546973. stm)

Retrieved on 2008-05-27[5] Sample, Ian. "The god of small things" (http:/ / www. guardian. co. uk/ weekend/ story/ 0,,2210858,00. html), The Guardian, November 17,

2007, weekend section.[6] The Cotham Grammar School building now houses Cotham School, a specialist performing arts school. (http:/ / www. cotham. bristol. sch.

uk/ index. php?option=com_content& task=view& id=1& Itemid=1)[7] "Higgs particle" (http:/ / www. britannica. com/ eb/ article-9040396/ Higgs-particle), Encyclopaedia Britannica, 2007.[8] P. W. Higgs Physics Letters 12 132 (1964).[9] P. W. Higgs Phys.Rev.Lett. 13 508 (1964).[10] Broken Symmetry and the Mass of Gauge Vector Mesons (http:/ / prola. aps. org/ abstract/ PRL/ v13/ i9/ p321_1)[11] Global Conservation Laws and Massless Particles (http:/ / prola. aps. org/ abstract/ PRL/ v13/ i20/ p585_1)[12] Physical Review Letters - 50th Anniversary Milestone Papers (http:/ / prl. aps. org/ 50years/ milestones#1964)[13] " Interview: the man behind the 'God particle' (http:/ / www. newscientist. com/ channel/ opinion/ mg19926732.

100-interview-the-man-behind-the-god-particle. html)", New Scientist 13 Sept., 2008, pp. 44–5[14] Key scientist sure "God particle" will be found soon (http:/ / www. reuters. com/ article/ scienceNews/ idUSL0765287220080407?sp=true)

Reuters news story. 7 April 2008.[15] Randerson, James (June 30, 2008). "Father of the 'God Particle'" (http:/ / www. guardian. co. uk/ science/ 2008/ jun/ 30/ higgs. boson. cern).

The Guardian. .[16] Rincon, Paul (24 July 2011) "Higgs boson 'hints' also seen by US lab" (http:/ / www. bbc. co. uk/ news/ science-environment-14266358)

BBC News. Retrieved 24 July 2011.[17] Highfield, Roger. "Prof Peter Higgs profile" (http:/ / www. telegraph. co. uk/ science/ science-news/ 3338770/ Prof-Peter-Higgs-profile.

html). The Telegraph. . Retrieved 16 May 2011.

Peter Higgs 68

[18] Rodgers, Peter. "The heart of the matter" (http:/ / www. independent. co. uk/ news/ science/ the-heart-of-the-matter-558435. html). TheIndependent. . Retrieved 16 May 2011.

[19] "Portrait of Peter Higgs by Ken Currie, 2010" (http:/ / www. tait. ac. uk/ Peter_Higgs_by_Ken_Currie. html). The Tait Institute. . Retrieved28 April 2011.

[20] Wade, Mike. "Portrait of a man at beginning of time" (http:/ / www. timesonline. co. uk/ tol/ news/ uk/ scotland/ article5835305. ece). TheTimes. . Retrieved 28 April 2011.

[21] "Great minds meet at portrait unveiling" (http:/ / www. ed. ac. uk/ news/ all-news/ higgs-portait-030309). The University of Edinburgh. .Retrieved 28 April 2011.

External links• Peter Higgs (http:/ / genealogy. math. ndsu. nodak. edu/ id. php?id=35098) at the Mathematics Genealogy Project.• Google Scholar (http:/ / scholar. google. com/ scholar?hl=en& lr=& q=author:PW+ author:Higgs& btnG=Search)

List of Papers by PW Higgs• A photograph of Peter Higgs (http:/ / www. particlephysics. ac. uk/ news/ picture-of-the-week/ picture-archive/

the-man-behind-the-higgs-particle. html), Photographs of Peter Higgs, June 2008 (http:/ / www. ph. ed. ac. uk/peter-higgs/ )

• The god of small things (http:/ / www. guardian. co. uk/ science/ 2007/ nov/ 17/ sciencenews. particlephysics) -An interview with Peter Higgs in The Guardian

• Peter Higgs: the man behind the boson (http:/ / physicsweb. org/ articles/ world/ 17/ 7/ 6) - An article in thePhysicsWeb about Peter Higgs

• Higgs v Hawking: a battle of the heavyweights that has shaken the world of theoretical physics (http:/ / web.archive. org/ web/ 20031116195026/ http:/ / millennium-debate. org/ ind3sept023. htm) - An article on the debatebetween Peter Higgs and Stephen Hawking about the existence of the Higgs boson

• My Life as a Boson (http:/ / wlap. physics. lsa. umich. edu/ umich/ mctp/ conf/ 2001/ sto2001/ higgs/ ) - ALecture by Peter Higgs available in various formats

• blog of an interview (http:/ / theatomsmashers. blogspot. com/ 2004/ 07/ peter-higgs-as-in-higgs-boson. html)• Physical Review Letters - 50th Anniversary Milestone Papers (http:/ / prl. aps. org/ 50years/ milestones#1964)• In CERN Courier, Steven Weinberg reflects on spontaneous symmetry breaking (http:/ / cerncourier. com/ cws/

article/ cern/ 32522)• Physics World, Introducing the little Higgs (http:/ / physicsworld. com/ cws/ article/ print/ 11353)• Englert-Brout-Higgs-Guralnik-Hagen-Kibble Mechanism on Scholarpedia (http:/ / www. scholarpedia. org/

article/ Englert-Brout-Higgs-Guralnik-Hagen-Kibble_mechanism)• History of Englert-Brout-Higgs-Guralnik-Hagen-Kibble Mechanism on Scholarpedia (http:/ / www. scholarpedia.

org/ article/ Englert-Brout-Higgs-Guralnik-Hagen-Kibble_mechanism_(history))• Sakurai Prize Videos (http:/ / www. youtube. com/ view_play_list?p=BDA16F52CA3C9B1D)

69

Appendix

List of particlesThis is a list of the different types of particles, known and hypothesized. For a chronological listing ofsubatomic particles by discovery date, see Timeline of particle discoveries.

This is a list of the different types of particles found or believed to exist in the whole of the universe. For individuallists of the different particles, see the individual pages given below.

Elementary particlesElementary particles are particles with no measurable internal structure; that is, they are not composed of otherparticles. They are the fundamental objects of quantum field theory. Many families and sub-families of elementaryparticles exist. Elementary particles are classified according to their spin. Fermions have half-integer spin whilebosons have integer spin. All the particles of the Standard Model have been observed, with the exception of theHiggs boson.

FermionsFermions have half-integer spin; for all known elementary fermions this is 1⁄2. All known fermions are Diracfermions; that is, each known fermion has its own distinct antiparticle. It is not known whether the neutrino is aDirac fermion or a Majorana fermion.[1] Fermions are the basic building blocks of all matter. They are classifiedaccording to whether they interact via the color force or not. In the Standard Model, there are 12 types of elementaryfermions: six quarks and six leptons.

Quarks

Quarks are the fundamental constituents of hadrons and interact via the strong interaction. Quarks are the onlyknown carriers of fractional charge, but because they combine in groups of three (baryons) or in groups of two withantiquarks (mesons), only integer charge is observed in nature. Their respective antiparticles are the antiquarkswhich are identical except for the fact that they carry the opposite electric charge (for example the up quark carriescharge +2⁄3, while the up antiquark carries charge −2⁄3), color charge, and baryon number. There are six flavors ofquarks; the three positively charged quarks are called up-type quarks and the three negatively charged quarks arecalled down-type quarks.

Quarks

Name Symbol Antiparticle Chargee

Mass (MeV/c2)

up u u +2⁄31.5–3.3

down d d −1⁄33.5–6.0

charm c c +2⁄31,160–1,340

strange s s −1⁄370–130

top t t +2⁄3169,100–173,300

List of particles 70

bottom b b −1⁄34,130–4,370

Leptons

Leptons do not interact via the strong interaction. Their respective antiparticles are the antileptons which areidentical except for the fact that they carry the opposite electric charge and lepton number. The antiparticle of theelectron is the antielectron, which is nearly always called positron for historical reasons. There are six leptons intotal; the three charged leptons are called electron-like leptons, while the neutral leptons are called neutrinos.

Leptons

Name Symbol Antiparticle Chargee

Mass (MeV/c2)

Electron e− e+ −1 0.511

Electron neutrino νe νe 0

Muon μ− μ+ −1 105.7

Muon neutrino νμ νμ 0 < 0.170

Tau τ− τ+ −1 1,777

Tau neutrino ντ ντ 0 < 15.5

BosonsBosons have integer spin. The fundamental forces of nature are mediated by gauge bosons, and mass is hypothesizedto be created by the Higgs boson. According to the Standard Model (and to both linearized general relativity andstring theory, in the case of the graviton) the elementary bosons are:

Name Symbol Antiparticle Charge (e) Spin Mass (GeV/c2) Interaction mediated Existence

Photon γ Self 0 1 0 Electromagnetism Confirmed

W boson W− W+ −1 1 80.4 Weak interaction Confirmed

Z boson Z Self 0 1 91.2 Weak interaction Confirmed

Gluon g Self 0 1 0 Strong interaction Confirmed

Higgs boson H0 Self 0 0 > 112 Mass Unconfirmed

Graviton G Self 0 2 0 Gravitation Unconfirmed

The graviton is added to the list although it is not predicted by the Standard Model, but by other theories in theframework of quantum field theory.The Higgs boson is postulated by electroweak theory primarily to explain the origin of particle masses. In a processknown as the Higgs mechanism, the Higgs boson and the other fermions in the Standard Model acquire mass viaspontaneous symmetry breaking of the SU(2) gauge symmetry. It is the only Standard Model particle not yetobserved (the graviton is not a Standard Model particle). The Minimal Supersymmetric Standard Model (MSSM)predicts several Higgs bosons. If the Higgs boson exists, it is expected to be discovered at the Large HadronCollider.

List of particles 71

Hypothetical particlesSupersymmetric theories predict the existence of more particles, none of which have been confirmed experimentallyas of 2011:

Superpartners

Superpartner Superpartnerof

Spin Notes

neutralino neutral bosons 1⁄2The neutralinos are superpositions of the superpartners of neutral Standard Model bosons: neutral higgs

boson, Z boson and photon.The lightest neutralino is a leading candidate for dark matter.

The MSSM predicts 4 neutralinos

chargino charged bosons 1⁄2The charginos are superpositions of the superpartners of charged Standard Model bosons: charged higgs

boson and W boson.The MSSM predicts two pairs of charginos.

photino photon 1⁄2Mixing with zino, neutral wino, and neutral Higgsinos for neutralinos.

wino, zino W± and Z0

bosons

1⁄2Charged wino mixing with charged Higgsino for charginos, for the zino see line above.

Higgsino Higgs boson 1⁄2For supersymmetry there is a need for several Higgs bosons, neutral and charged, according with their

superpartners.

gluino gluon 1⁄2Eight gluons and eight gluinos.

gravitino graviton 3⁄2Predicted by Supergravity (SUGRA). The graviton is hypothetical, too – see next table.

sleptons leptons 0 The superpartners of the leptons (electron, muon, tau) and the neutrinos.

sneutrino neutrino 0 Introduced by many extensions of the Standard Model, and may be needed to explain the LSND results.A special role has the sterile sneutrino, the supersymmetric counterpart of the hypothetical right-handed

neutrino, called sterile neutrino

squarks quarks 0 The stop squark (superpartner of the top quark) is thought to have a low mass and is often the subject ofexperimental searches.

Note: Just as the photon, Z boson and W± bosons are superpositions of the B0, W0, W1, and W2 fields – the photino,zino, and wino± are superpositions of the bino0, wino0, wino1, and wino2 by definition.No matter if you use the original gauginos or this superpositions as a basis, the only predicted physical particles areneutralinos and charginos as a superposition of them together with the Higgsinos.Other theories predict the existence of additional bosons:

Other hypothetical bosons and fermions

Name Spin Notes

Higgs 0 Has been proposed to explain the origin of mass by the spontaneous symmetry breaking of the SU(2) x U(1) gaugesymmetry.

SUSY theories predict more than one type of Higgs boson

graviton 2 Has been proposed to mediate gravity in theories of quantum gravity.

graviscalar 0 Also known as radion

graviphoton 1 Also known as gravivector[2]

axion 0 A pseudoscalar particle introduced in Peccei-Quinn theory to solve the strong-CP problem.

List of particles 72

axino 1⁄2Superpartner of the axion. Forms, together with the saxion and axion, a supermultiplet in supersymmetric extensions

of Peccei-Quinn theory.

saxion 0

branon ? Predicted in brane world models.

dilaton 0 Predicted in some string theories.

dilatino 1⁄2Superpartner of the dilaton

X and Y bosons 1 These leptoquarks are predicted by GUT theories to be heavier equivalents of the W and Z.

W' and Z'bosons

1

magnetic photon ?

majoron 0 Predicted to understand neutrino masses by the seesaw mechanism.

majoranafermion

1⁄2 ; 3⁄2

?...

Gluinos, neutralinos, or other

Mirror particles are predicted by theories that restore parity symmetry.Magnetic monopole is a generic name for particles with non-zero magnetic charge. They are predicted by someGUTs.Tachyon is a generic name for hypothetical particles that travel faster than the speed of light and have an imaginaryrest mass.Preons were suggested as subparticles of quarks and leptons, but modern collider experiments have all but ruled outtheir existence.Kaluza-Klein towers of particles are predicted by some models of extra dimensions. The extra-dimensionalmomentum is manifested as extra mass in four-dimensional space-time.

Composite particles

HadronsHadrons are defined as strongly interacting composite particles. Hadrons are either:• Composite fermions, in which case they are called baryons.• Composite bosons, in which case they are called mesons.Quark models, first proposed in 1964 independently by Murray Gell-Mann and George Zweig (who called quarks"aces"), describe the known hadrons as composed of valence quarks and/or antiquarks, tightly bound by the colorforce, which is mediated by gluons. A "sea" of virtual quark-antiquark pairs is also present in each hadron.

List of particles 73

Baryons (fermions)

A combination of three u, d or s-quarks with atotal spin of 3⁄2 form the so-called baryon

decuplet.

Proton quark structure: 2 up quarks and 1 down quark.

For a detailed list, see List of baryons.

Ordinary baryons (composite fermions) contain three valencequarks or three valence antiquarks each.

• Nucleons are the fermionic constituents of normal atomicnuclei:

• Protons, composed of two up and one down quark (uud)• Neutrons, composed of two down and one up quark (ddu)

• Hyperons, such as the Λ, Σ, Ξ, and Ω particles, which containone or more strange quarks, are short-lived and heavier thannucleons. Although not normally present in atomic nuclei, theycan appear in short-lived hypernuclei.

• A number of charmed and bottom baryons have also beenobserved.

Some hints at the existence of exotic baryons have been foundrecently; however, negative results have also been reported. Theirexistence is uncertain.

• Pentaquarks consist of four valence quarks and one valenceantiquark.

Mesons (bosons)

Mesons of spin 0 form a nonet

For a detailed list, see List of mesons.

Ordinary mesons are made up of a valence quark and a valenceantiquark. Because mesons have spin of 0 or 1 and are not themselveselementary particles, they are composite bosons. Examples of mesonsinclude the pion, kaon, the J/ψ. In quantum hadrodynamic models,mesons mediate the residual strong force between nucleons.

At one time or another, positive signatures have been reported for all ofthe following exotic mesons but their existence has yet to beconfirmed.

• A tetraquark consists of two valence quarks and two valenceantiquarks;

• A glueball is a bound state of gluons with no valence quarks;• Hybrid mesons consist of one or more valence quark-antiquark pairs and one or more real gluons.

List of particles 74

Atomic nuclei

A semi-accurate depiction of the helium atom. In the nucleus, the protons are in redand neutrons are in purple. In reality, the nucleus is also spherically symmetrical.

Atomic nuclei consist of protons andneutrons. Each type of nucleus contains aspecific number of protons and a specificnumber of neutrons, and is called a nuclideor isotope. Nuclear reactions can change onenuclide into another. See table of nuclidesfor a complete list of isotopes.

Atoms

Atoms are the smallest neutral particles intowhich matter can be divided by chemicalreactions. An atom consists of a small,heavy nucleus surrounded by a relativelylarge, light cloud of electrons. Each type ofatom corresponds to a specific chemicalelement. To date, 118 elements have beendiscovered, while only the first 112 havereceived official names. Refer to theperiodic table for an overview.

The atomic nucleus consists of protons andneutrons. Protons and neutrons are, in turn, made of quarks.

Molecules

Molecules are the smallest particles into which a non-elemental substance can be divided while maintaining thephysical properties of the substance. Each type of molecule corresponds to a specific chemical compound. Moleculesare a composite of two or more atoms. See list of compounds for a list of molecules.

Condensed matterThe field equations of condensed matter physics are remarkably similar to those of high energy particle physics. As aresult, much of the theory of particle physics applies to condensed matter physics as well; in particular, there are aselection of field excitations, called quasi-particles, that can be created and explored. These include:• Phonons are vibrational modes in a crystal lattice.• Excitons are bound states of an electron and a hole.• Plasmons are coherent excitations of a plasma.• Polaritons are mixtures of photons with other quasi-particles.• Polarons are moving, charged (quasi-) particles that are surrounded by ions in a material.• Magnons are coherent excitations of electron spins in a material.

List of particles 75

Other• An anyon is a generalization of fermion and boson in two-dimensional systems like sheets of graphene which

obeys braid statistics.• A plekton is a theoretical kind of particle discussed as a generalization of the braid statistics of the anyon to

dimension > 2.• A WIMP (weakly interacting massive particle) is any one of a number of particles that might explain dark matter

(such as the neutralino or the axion).• The pomeron, used to explain the elastic scattering of Hadrons and the location of Regge poles in Regge theory.• The skyrmion, a topological solution of the pion field, used to model the low-energy properties of the nucleon,

such as the axial vector current coupling and the mass.• A genon is a particle existing in a closed timelike world line where spacetime is curled as in a Frank Tipler or

Ronald Mallett time machine.• A goldstone boson is a massless excitation of a field that has been spontaneously broken. The pions are

quasi-Goldstone bosons (quasi- because they are not exactly massless) of the broken chiral isospin symmetry ofquantum chromodynamics.

• A goldstino is a Goldstone fermion produced by the spontaneous breaking of supersymmetry.• An instanton is a field configuration which is a local minimum of the Euclidean action. Instantons are used in

nonperturbative calculations of tunneling rates.• A dyon is a hypothetical particle with both electric and magnetic charges• A geon is an electromagnetic or gravitational wave which is held together in a confined region by the

gravitational attraction of its own field energy.• An inflaton is the generic name for an unidentified scalar particle responsible for the cosmic inflation.• A spurion is the name given to a "particle" inserted mathematically into an isospin-violating decay in order to

analyze it as though it conserved isospin.• What is called "true muonium", a bound state of a muon and an antimuon, is a theoretical exotic atom which has

never been observed.

Classification by speed• A tardyon or bradyon travels slower than light and has a non-zero rest mass.• A luxon travels at the speed of light and has no rest mass.• A tachyon (mentioned above) is a hypothetical particle that travels faster than the speed of light and has an

imaginary rest mass.

References[1] B. Kayser, Two Questions About Neutrinos, arXiv:1012.4469v1 [hep-ph] (2010).[2] R. Maartens (2004). Brane-World Gravity (http:/ / www. emis. de/ journals/ LRG/ Articles/ lrr-2004-7/ download/ lrr-2004-7BW. pdf). 7. 7. .

Also available in web format at http:/ / www. livingreviews. org/ lrr-2004-7.

• C. Amsler et al. (Particle Data Group) (2008). "Review of Particle Physics". Physics Letters B 667 (1-5): 1.Bibcode 2008PhLB..667....1P. doi:10.1016/j.physletb.2008.07.018. (All information on this list, and more, can befound in the extensive, biannually-updated review by the Particle Data Group (http:/ / pdg. lbl. gov))

Article Sources and Contributors 76

Article Sources and ContributorsParticle physics  Source: http://en.wikipedia.org/w/index.php?oldid=461842377  Contributors: 128.12.93.xxx, 142.58.249.xxx, 64.26.98.xxx, APH, AgadaUrbanit, Agerom, Ahoerstemeier,Aknochel, Alansohn, Allstarecho, Almostcrime, AndreasJS, Andycjp, Archer7, Ark, Aroodman, Arthena, Atlant, Austin Maxwell, Awmarcz, AxelBoldt, Bambaiah, Bamkin, Barbara Shack,Battlemage, Bdesham, Bennylin, Bevo, Bggoldie, Bm gub, BobertWABC, Bobo192, Bodhitha, Boing! said Zebedee, Boud, Brandonlovescrashincastles, Brews ohare, BurtPeck, CRGreathouse,CRKingston, CWii, CYD, Calmypal, Caltas, CambridgeBayWeather, Can't sleep, clown will eat me, Celithemis, Ch2pgj, Chenyu, CimanyD, Cjc38, CloudNineAC, Complex (de), Comrade42,Conversion script, Csgwon, Cybercobra, DHN, DV8 2XL, Dauto, Deglr6328, Dev 176, Diligent Terrier, Discospinster, Djegan, Docu, Dominick, Donarreiskoffer, Donzzz77, Drphilharmonic,Ebehn, Edward Z. Yang, El C, El Snubbe, Ellywa, Elodzinski, Eloquence, EmanCunha, Emijrp, Eog1916, Erwinrossen, FT2, Falcon8765, Falconkhe, Favonian, Fieldday-sunday, Fruge, GaiusCornelius, Gareth Owen, Gary King, Gbrandt, GeorgeLouis, Ghalhud, Giftlite, Glenn, Gnixon, Gnomon Kelemen, Goodnightmush, Goudzovski, Graham87, Haxwell, Hdeasy, Head, Headbomb,Hectorthebat, Henry W. Schmitt, Hepforever, Hfastedge, Howdychicken, Howie Goodell, IOPhysics, Ilmari Karonen, Immunize, Inwind, Iridescent, Ironboy11, Isnow, Ixfd64, J.delanoy, JRRTrollkien, JaGa, Jagged 85, Jameskeates, Jamesontai, JamieS93, Jgwacker, JimVC3, Jimbill4321, Joe N, Joe iNsecure, Jomoal99, JonasRH, Joshmt, Jpowell, Jung dalglish, Jxzj, Kakofonous,Karol Langner, Kbrose, Kenneth M Burke, Khcf6971, Kocio, Korath, Kozuch, Kuru, Kurzon, LX, Langsytank, Larry Sanger, Laussy, Le sacre, Lee Daniel Crocker, Lightdarkness, Lightmouse,Ling.Nut, Lmatt, Looxix, Lor772, Lseixas, Lumidek, Lupin, MER-C, MK8, Mako098765, Marcus Qwertyus, Master Jay, Mato, Matt Crypto, Matt Gies, Matthew Woodcraft, Mattmartin,Maurreen, Mav, Mayumashu, Mcneile, Md7t, Melchoir, Mermaid from the Baltic Sea, Metrictensor, Mets501, Michael Hardy, MichaelMaggs, Micraboy, Mignon, Mike2vil, MonoAV,MoogleEXE, Mouse7525, Mpatel, Mullactalk, Munkay, Mxn, Navasj, NellieBly, News0969, Novacatz, NuclearWinner, Ohconfucius, Ohwilleke, Oldnoah, Olexandr Kravchuk, Olhp,OpenToppedBus, Orion11M87, Orpheus, Oxymoron83, Paine Ellsworth, Palfrey, Pandacomics, Party, Patrick, Pcd72, Pchapman47879, Penarestel, Pet3r, PhoenixFlentge, PhySusie, Phys,Physics, Physicsdavid, Physis, Planlips, Plastadity, Poopfacer, PranksterTurtle, QFT, RE, Ragesoss, Rangoon11, Raphtee, Raul654, Ravi12346, Rclsa, Redvers, Res2216firestar, Rholton, RichFarmbrough, Rje, Rl, Roadrunner, Rorro, Ryan Postlethwaite, SCZenz, Saeed.Veradi, SaltyBoatr, Sanders muc, SarahLawrence Scott, Savidan, Scottfisher, Selkem, Shawn in Montreal, Sillyrabbit, SimonMayer, SimonP, Sjakkalle, Smarcus, Snigbrook, Sodium, Someguy1221, Sonicology, Srleffler, Stephenb, Steve Quinn, SwordSmurf, TallMagic, Techraj, The Epopt, Tpbradbury,Trecool12, Trelvis, Truthnlove, Tycho, UncleDouggie, UninvitedCompany, Urvabara, Van helsing, VanishedUser314159, Velella, VictorFlaushenstein, Vishnava, Voidxor, Voyajer, Wavelength,Who, WikHead, Will Gladstone, Witguiota, Wolftengu, XJamRastafire, Ylai, Zanzerjewel, Ъыь, 404 anonymous edits

Standard Model  Source: http://en.wikipedia.org/w/index.php?oldid=462915508  Contributors: A. di M., APH, Adam Krellenstein, Addshore, Afteread, AgadaUrbanit, Agasicles, Agasides,Aknochel, Alan Liefting, Alansohn, Alinor, Aliotra, Alison, AmarChandra, Andre Engels, Andycjp, AnonMoos, Aoosten, Arivero, AugPi, Awren, AxelBoldt, Axl, Bakken, Bambaiah, Bamkin,Barak Sh, Bassbonerocks, Bdijkstra, BenRG, Benbest, Bender235, Beta Orionis, Bevo, Bodhitha, Bookalign, Bovineone, Brews ohare, Brim, Brockert, Bryan Derksen, Bubba73, Bytbox,C0nanPayne, CYD, Caco de vidro, CattleGirl, Chris the speller, ChristopherWillis, Complexica, Craig Bolon, Crazz bug 5, Crum375, Cybercobra, D-Notice, DWHalliday, DadaNeem,Daniel.Cardenas, DannyWilde, Dauto, Dave1g, David Barnard, David Schaich, David spector, Dbenbenn, Dbraize, Deepmath, DerNeedle, Derek Ross, Dextrose, Dfan, Diagramma Della Verita,Djr32, Dmmaus, Dratman, Drhex, Drrngrvy, Drxenocide, Dstudent, Dv82matt, Dysepsion, Edsegal, Edward, Eeekster, Egg, Ekjon Lok, El C, Elsweyn, Epbr123, Ernsts, Escalona, FT2, Faethon,Faethon34, Faethon36, Fences and windows, Fogger, FrankTobia, Gary King, Gatortpk, Geremia, Giftlite, Glenn, Gnixon, Goop Goop, Goudzovski, Gparker, Gscshoyru, Guarracino, Guy Harris,H2g2bob, HEL, Hal peridol, Hans Dunkelberg, Haoherb428, Harp, Harrigan, Headbomb, Herbee, Hexane2000, Hirak 99, HorsePunchKid, HungarianBarbarian, IanOfNorwich, Icairns, Icalanise,Iomesus, Isis, Isocliff, Itinerant1, J Milburn, J.delanoy, JLaTondre, JabberWok, Jacksonwalters, Jagged 85, JamesAM, JarahE, JeffBobFrank, Jeffq, Jeodesic, Jessemv, Jgwacker, Jim E. Black,Jmnbatista, Joshmt, Jrf, Jrtayloriv, Jrw@pobox.com, JulesH, Julesd, Kacser, Kate, KathrynLybarger, Kenmint, Kocio, Laurascudder, LeYaYa, Len Raymond, Leszek Jańczuk, Likebox, LilHelpa,Linas, Lomn, Looxix, Lottamiata, MJ94, Macumba, Maldmac, Mastertek, Melchoir, Metacomet, Michael C Price, Michael Hardy, Michaelbusch, Mindmatrix, Mjamja, Monedula, Moose-32,Mosaffa, MovGP0, Mpatel, Mxn, Naraht, Nozzer42, Nurg, Ohwilleke, Ordovico, Orion11M87, Orionus, Paine Ellsworth, Patrick, Patrickwooldridge, Pharotic, Phatom87, Phr, Phys, Physicistbrazuca, Physics therapist, Populus, QFT, QMarion II, Qwertyca, R.e.b., RG2, Ram-Man, Rama, Raven in Orbit, Rbj, Reddi, Rjwilmsi, Roadrunner, Robdunst, Roscoe x, SCZenz, Schucker,Seaphoto, SebastianHelm, Securiger, Setanta747 (locked), Setreset, SheepNotGoats, Sheliak, Silly rabbit, Sligocki, Soarhead77, Sonjaaa, Stannered, Stephen Poppitt, Steve Quinn, Stevertigo,Stillnotelf, Stormymountain, Superm401, Superwj5, Suslindisambiguator, Swamy g, TPickup, Tanner Swett, Tarcieri, Tariqhada, Template namespace initialisation script, TenOfAllTrades,Tetracube, Texture, That Guy, From That Show!, The Anome, The Transliterator, Thunderboltz, TimBentley, Tirebiter78, Tom Lougheed, TriTertButoxy, Truthnlove, Twas Now,UnitedStatesian, UniversumExNihilo, Van helsing, Verdy p, Vessels42, Voorlandt, VoxMoose, WJBscribe, Waggers, Wilhelm-physiker, Wing gundam, Wtmitchell, Wwheaton, Xerxes314,Xezbeth, YellowMonkey, Yevgeny Kats, Youandme, ZakMarksbury, 332 anonymous edits

Supersymmetry  Source: http://en.wikipedia.org/w/index.php?oldid=463089994  Contributors: Acjohnson55, Aknochel, Ancheta Wis, Andre Engels, Anville, Arivero, Barak Sh, BenRG, Bhny,Blaxthos, Bodera, Bryan Derksen, C9, CES1596, Cadmasteradam, Can't sleep, clown will eat me, Cgingold, Chaos, Charles Matthews, Charleswestbrook, Chessmaster7m, Cless Alvein,Closedmouth, Complexica, Crum375, Cuboidal, DO'Neil, Dan Gluck, Ddimensões, Deglr6328, Djloststylez, Drrngrvy, Duk, Eddie Nixon, Edward, El C, Epolk, F Notebook, Ferkelparade,Francescog, Fropuff, Gagoga ju, Gary King, Giftlite, Gil987, Girl Scout cookie, Gparker, Gsard, Gus Polly, HaloStereo1, Headbomb, IMSoP, Isocliff, J.christianson, JarahE, Jcpilman, Jeandré duToit, Jgwacker, Jordan14, Josiah Rowe, Jpod2, KFP, Kawakameha, Kborland, Kevin Hickerson, Killing Vector, Koeplinger, Kostisl, Kurochka, Lambiam, LiDaobing, Lmatt, LostLeviathan,Lumidek, MFH, Maarten van Vliet, Maliz, Mastertek, Maurice Carbonaro, Maury Markowitz, Maxim Leyenson, Maxim Razin, Maximus Rex, Mdanziger, Mgnbar, Michael C Price, MichaelHardy, Mira, Mishas42, Monedula, Mor, Moyogo, Mpatel, Mporter, Nn123645, Nonnormalizable, Nowhither, Ohwilleke, Pearle, Pharotic, Phys, PhysPhD, Plumpurple, Ptrslv72, Puzl bustr,QFT, R.e.b., RG2, RJFJR, Radagast83, Raul654, Reaverdrop, Rich Farmbrough, Rjwilmsi, Roadrunner, Robma, Roybb95, Ruff ilb, Rursus, Salgueiro, Sam Hocevar, Scrabby, SeventyThree,Sheliak, Smack, Solarapex, Stevertigo, Susy is it, Taw, Ted BJ, That Guy, From That Show!, TheMaster42, Theresa knott, TimothyRias, Tktktk, TriTertButoxy, Tweet Tweet, Unconcerned,VermillionBird, Vyroglyph, Wangjiaji, Wavelength, WikHead, Wtmitchell, Xerxes314, Xiaphias, Yevgeny Kats, Zahd, Zentropa77, 197 anonymous edits

Elementary particle  Source: http://en.wikipedia.org/w/index.php?oldid=462590190  Contributors: 217.126.156.xxx, Abdullais4u, Acalamari, Acroterion, Albert Einsteins pipe, Alison,Amazins490, Ancheta Wis, Anthony, Antixt, Anythingyouwant, Appraiser, ArglebargleIV, Arthur Rubin, AugPi, Azmaverick623, Bamkin, Bci2, Bevo, Beyond My Ken, Big55e, Blah28948,Blaxthos, Bodnotbod, Bomac, BraneJ, Brianjd, Bryan Derksen, Bvcrist, CYD, Cactus.man, Chaos, Chris the speller, Cockshut12345, Complexica, Conversion script, Courcelles, Craig Stuntz,Cybercobra, DV8 2XL, DannyWilde, DavidCary, Dekaels, Dekisugi, Dhatfield, Dirac1933, Dmr2, Dna-webmaster, Donarreiskoffer, DraakUSA, Draco 2k, Drphilharmonic, EPM, Edwardlalone,Eurobas, Fbjon, Fordmadoxfraud, Funky Fantom, Furrykef, Garry Denke, George Rodney Maruri Game, Ggonnell, Giftlite, Girl Scout cookie, Glenn, Goplat, GrGBL, Greydream, Guzman.c,H3llkn0wz, Hairy Dude, Haselhurst, Headbomb, Heron, Hidaspal, I hate whitespace, Icalanise, Isnow, J Milburn, Jimduck, Jmorris84, Jonhall, Joriki, Joshua Issac, Jynus, Kate, Kay Dekker, Keng6, Kenneth M Burke, Kozuch, Laplace's Demon, Larsobrien, Leaverward, Let'sBuildTheFuture, Ligulem, Lk69, Lmatt, Looxix, Lord Hawk, Markinvancouver, Mav, Maxtitan, Micah.yannatos1,MichaelMaggs, MikeBaharmast, Mikez, Miserlou, Mnkyman, Monedula, Mormegil, MovGP0, Mpatel, Mrcoolbp, Mthorndill, Mxn, Negovori, Neonumbers, Newone, Ohwilleke, Onesius,Orion11M87, Oyvind, Palica, Papadopc, Paradoxalterist, Patrick, Perfectapproach, Phazvmk, Physics therapist, Quantumor, Qutezuce, R'n'B, RJHall, RP459, Reddi, Rightly, Robo37, RolteVolte,RoyBoy, SCZenz, Sadi Carnot, Saintlucifer2008, SarahLawrence Scott, SchmittM, Sgreddin, Silly rabbit, Simetrical, Sonicology, Spaceawesome, Srleffler, Ssr, Stephenb, Steve Quinn,Stevertigo, Sthyne, Strait, Svick, Templatehater, Think!97, Timb66, Timwi, Tktktk, Tobi - Tobsen, TomTheHand, TomasBat, Truthnlove, Urvabara, Vibritannia, WhiteHatLurker, XJamRastafire,Xerxes314, Yahia.barie, Yakiv Gluck, Александър, 147 ,ליאור anonymous edits

Boson  Source: http://en.wikipedia.org/w/index.php?oldid=458547350  Contributors: 217.126.156.xxx, Abdullais4u, Abtvctkto61, Ace of Spades, Aldie, Altenmann, Alxndr, Andres, Andrewa,Anonymous Dissident, Anthony Appleyard, Antixt, Asnr 6, Beland, Ben-Zin, Benbest, Bertrem, Bkalafut, Bradenripple, BriEnBest, Buckyboy314, CAkira, CYD, Candamir, Cenarium,Complexica, Conversion script, CosineKitty, Cructacean, Dan Gluck, DanielCD, Danjel, Danthewhale, Dicklyon, Discospinster, Drxenocide, Enchanter, Enormousdude, Epastore, Evil saltine,Fropuff, Frozenport, Giftlite, Gilliam, Glenn, Gravitivistically, GregorB, H2g2bob, Hadal, Hal peridol, Headbomb, Hidaspal, Hqb, Ianji, Icairns, Inertiatic076, Iridescent, Jack who built thehouse, Jensbn, Jeraaldo, Jim E. Black, Jlandahl, Jossi, Kaihsu, Kbdank71, Kbh3rd, Kingpin13, La goutte de pluie, Leoadec, Lisiate, Looxix, MK8, MagnaMopus, Master Justin, Mathninja, Mbell,Merovingian, Mgurgan, Michael Hardy, MichaelMaggs, Mike2vil, Minami Kana, Mormegil, Mpatel, Nakos2208, NawlinWiki, Nilradical, Noisy, Nsajjansajja, OlEnglish, Orionus, Ornil,Ouchitburns, Owhanow, Palica, Panoramix, Pharotic, Philvarner, Phys, Pkoppenb, Pyg, QFT, R'n'B, RadicalOne, Radon210, Roadrunner, Robhenry9, Robinh, Rocastelo, Rorro, RuneSylvester,Salsb, Schneelocke, Scienz Guy, Senator Palpatine, Srleffler, Stebbins, Strait, Sunborn, T-dot, Tarotcards, The Anome, The Wild Falcon, The1physicist, TheEditrix2, Tim Starling, Tpvibes,Uberdude85, UltraHighVacuum, VashiDonsk, Voyajer, W.F.Galway, Ward3001, Welsh, WhiteHatLurker, Wikeepedian, Wikiborg, Wragge, Zfr, Zzedar, 158 anonymous edits

Higgs boson  Source: http://en.wikipedia.org/w/index.php?oldid=464531898  Contributors: -dennis-, 1ForTheMoney, A Man In Black, A. di M., ABF, Aardvark23, Abdullais4u, Adrideba, Aesir.le, Aknochel, Alansohn, Allstarecho, Altenmann, Alyjack, AnOddName, Anaxial, AndersFeder, Andrew c, AndrewN, Andrius.v, Angelo souti, AnonMoos, Anonymi, Antixt, Archelon, Art LaPella, Artur80, Asmeurer, Atomicthumbs, AxelBoldt, Baad, Bambaiah, Bbbl67, Bcody80, Bcorr, Ben MacDui, BenRG, Bender235, Benjiboy187, Benplowman, Betterusername, Bevo, Bhny, Big Brother 1984, Biker Biker, Bjankuloski06en, BobertWABC, Bodhitha, Bondegezou, Bookofjude, Boson15, Brainssturm, Brian Fenton, Brians, Britannica, Bryan Derksen, Bubba73, BullRangifer, Buster79, C S, CIS, CYD, Cadmasteradam, Caknuck, Calmer Waters, CamB424, CamB4242, CesarB, Cgd8d, Cgwaldman, CharlesC, Chetvorno, ChiZeroOne, Chreod, Christopher Thomas, Chuckupd, Cinkcool, ClaudeMuncey, Closedmouth, Comet Tuttle, Consumed Crustacean, Cructacean, D'Agosta, DBGustavson, DKqwerty, DMurphy, Daniel C, DannyDaWriter, Dante Alighieri, Dauto, Dave3457, David spector, Dbachmann, DeadlyMETAL, Deceglie, Dekker451, Dirac1933, Dirkbb, Discospinster, Diza, Donarreiskoffer, DrGaellon, Dragon of the Pants, Dratman, Drcooljoe, Drmies, DÅ‚ugosz, EchetusXe, Edderso, Edward, Ehn, Eikern, El C, ElfQrin, Eliga, Endersdouble, Epastore, Er ouz, Eritain, Ernsts, Ervin Goldfain, Escape Orbit, Excirial, Fatram, Fiziker, Fleisher, Fleminra, Flyguy649, Foobar, Foober, Foonle77, Fotoni, Frglee, Frymaster, Fæ, GDallimore, Gaurav, Giandrea, Giftlite, Gil987, Giuliopp, Gobbledygeek, Goethean, Golbez, Goudzovski, GregorB, Gurch, Gwib, Hadal, Hairy Dude, Harold f, Harp, Headbomb, Hellbus, Herbee, Herk1955, Heron, Higgshunter, Hippypink, I hate whitespace, Icairns, Iknowyourider, Ilmari Karonen, Impunv, Infestor, Irenan, Itinerant, Itinerant1, Iwpg, J M Rice, J mcandrews, J.delanoy, JCSantos, JTiago, JabberWok, Jacques Antoine, Jagged 85, Jason Quinn, JasonAQuest, Jc odcsmf, Jde123, Jdigitalbath, Jehochman, Jezzabr, Jfromcanada, Jgwacker, Jimtpat, Jkl, JohnArmagh, Johnflux, Jomoal99, JonathanDP81, Jonburchel, Jonbutterworth, Jor63, Joriki, JorisvS, Josh Cherry, Jpod2, Jtuggle, Justinrossetti, KHamsun, Kaihsu, KapilTagore, Kbdank71, Kbk, Kborland, Keith-264, Kencf0618, Kendrick7, Kenneth Dawson, Kgf0, Koavf, Kocio,

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Higgs mechanism  Source: http://en.wikipedia.org/w/index.php?oldid=464418906  Contributors: AVM, Aknochel, Algebraist, Ancheta Wis, Auntof6, Bakken, Bambaiah, Barak Sh, Bender235,Benji1986, Benzh, Beth Ann Lindstrom, BoLingua, Brews ohare, CBuiltother, CYD, Caco de vidro, Calwiki, Capricorn42, Cerlbar, Charles Matthews, Chris the speller, Christopher Thomas,Ciphers, Ckatz, Crowsnest, Cuzkatzimhut, Darsie, David Schaich, Difty, Doradus, Drmies, Duncan.france, Długosz, Edcolins, Epbr123, Eric Drexler, Felix0411, Finell, Galaxydraem, Gareth EKegg, General Epitaph, Giftlite, Gillleke, Haydarhan, Headbomb, Hekerui, Henry Delforn, Herbee, JacquesPHI, JohnWilliams, Jpod2, Kelly Martin, Koavf, Ldussan, Likebox, LilHelpa, Linas,LokiClock, LoserJoke, Lumidek, Maksim-e, Markdroberts, MarsRover, Mary at CERN, Mat cross, Meco, Meier99, Michael C Price, Michael Hardy, Moose-32, Myasuda, O.anatinus, OlegAlexandrov, Pac72, Palfrey, Phys, PhysicsAboveAll, Quibik, RickyCayley, Rjwilmsi, Roadrunner, Saintswithin, Sbisolo, Strait, Tetracube, TheAMmollusc, Tide rolls, Triple333, Viriditas, WestBrom 4ever, Wwheaton, Xxanthippe, Yeroretep, Yoavd, 181 anonymous edits

Large Hadron Collider  Source: http://en.wikipedia.org/w/index.php?oldid=464565605  Contributors: (jarbarf), -Majestic-, 02millers, 03md, 123smellmyfeet, 1dragon, 2bornot2b, 343GuiltySpark343, 4johnny, 842U, 84user, 99chromehead, A. di M., A.R., A3RO, AMK1211, Abdullais4u, AbhishekSinghRana, AcademyAD, Ace111, Acepectif, Acroterion, Adambro, Addw, Adejam, Adhalanay, Adilch, Adj08, Advertiseo, Aetheling1125, Afowler, AgadaUrbanit, Ageekgal, Ahoerstemeier, Ajeetkumar81, Aka042, Aknochel, Al Farnsworth, Alamgir, Alaniaris, Alansohn, Alberto da Calvairate, Alderepas, Aldnonymous, Alexgenaud, Alexius08, Ali@gwc.org.uk, Amaurea, Amire80, Amorymeltzer, Andefs, AndersFeder, Andre Engels, Andre.holzner, Andrewlp1991, Andrius.v, Andy M. 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Article Sources and Contributors 78

Yellowdesk, Yhkhoo, Ylai, Yuefairchild, ZZ9pluralZalpha, Zaak, Zargulon, Zestofalemon, Zidonuke, Zimbabweed, Zomglolwtfzor, Zonk43, Zsinj, Ztbbq, Ztobor, Zucchinidreams, Zykure,Zythe, 1644 anonymous edits

Peter Higgs  Source: http://en.wikipedia.org/w/index.php?oldid=464080346  Contributors: 28421u2232nfenfcenc, AVM, Abanima, Ahoerstemeier, Ancheta Wis, Anterior99, Badger Drink, BenMacDui, Bender235, Benjaminevans82, Blu Matt, Bogdangiusca, Brad7777, Brighterorange, CJLL Wright, CRKingston, Charles Matthews, Coyets, Curps, D6, Dan berliner,DancingPhilosopher, David Gerard, David from Downunder, Dcoetzee, Dirac1933, Dottydotdot, Duendeverde, Emerson7, Ephraim33, Eric Kvaalen, Fanica1995, Flaming Ferrari, Fragglet,G716, Gareth Owen, Gilliam, Gronteam, Headbomb, Herbee, Hugo999, Ian Page, Ironboy11, Irsample, Isidore, JamesBWatson, Jamiemaloneyscoreg, Jandalhandler, Japanese Searobin, Jaraalbe,Jewce, KTC, Khukri, Laputian, Mary at CERN, Maybeheard21, Memming, Moose-32, OoberMick, Pamri, Pdfpdf, Phe, RS1900, Random18, Ratarsed, Rdblnwr, Richard L. Peterson, RodC,Rsabbatini, SCZenz, Salsb, Scythia, Simeon, SlimVirgin, Smallweed, Snowmanradio, Snufking, StewartMH, SylviaStanley, Taagane19, Tassedethe, The wub, Timrollpickering, Tpbradbury,Travelbybus, Urvabara, Vernon39, Woohookitty, XJamRastafire, YUL89YYZ, 61 anonymous edits

List of particles  Source: http://en.wikipedia.org/w/index.php?oldid=463662603  Contributors: 041744, 112358sam, 49, A. di M., Adam Krellenstein, Adanadhel, Aegnor.erar, Ahoerstemeier,Alensha, Anaxial, Anonymous Dissident, Antixt, Arch dude, ArnoldReinhold, Asbestos, Atomic7732, AubreyEllenShomo, Audrius u, AxelBoldt, Axelfoley12, Axl, Azcolvin429, Bambaiah,Bazza 7, Bdesham, Betacommand, Bevo, Bissinger, Blennow, Bluetryst, Bodhitha, CBDunkerson, CalamusFortis, Cbuckley, Cedrus-Libani, Chaos, Charles Matthews, CityOfSilver,CommonsDelinker, Count Iblis, Cstmoore, D6, Dan Guan, Danny, DannyWilde, David Ko, Derdeib, Diffequa, Dirac1933, Discospinster, Docu, Donarreiskoffer, Dratman, Dstary, Duncan.france,Eddideigel, El C, Elmoosecapitan, EmilJ, Ems57fcva, Ernsts, Escalona, Evercat, Explicit, Favonian, Figureskatingfan, Flewis, Flickboy, FrozenMan, Gbrandt, Giftlite, Goudzovski, GraphiteElbow, GregorB, HESUPERMAN, Hamish a e fowler, Happy-melon, Hayne, Headbomb, Herbee, Hydrargyrum, Icairns, Icalanise, Inner Earth, Iridescent, Ishvara7, JLM, JPMasseo, JRSpriggs,JarlaxleArtemis, Jcimorra, Jeremy Henty, Jitse Niesen, Jmrowland, John, Johnman239, Jojhutton, Joshmt, Juancnuno, KDesk, Karam.Anthony.K, Khazar, Krash, L Kensington, LFaraone,Laurascudder, Lexicon, Linas, Lseixas, Mac Davis, Master of Puppets, Maurice Carbonaro, McGeddon, Merovingian, Minghong, Mysidia, Nikai, Nolimitownass, OlEnglish, Oleg Alexandrov,OliverHarris, OllieFury, Omicronpersei8, Paolo.dL, Pengo, Physicist, Physicistjedi, Poulpy, Quadell, Quibik, Quilbert, Qutezuce, Qwyrxian, RJFJR, RSStockdale, Radical Mallard, Raistuumum,RandorXeus, Raul654, RetiredUser2, Rjwilmsi, Rmhermen, Rotiro, SCZenz, Sadisticsuburbanite, Salsa Shark, Salsb, Sanders muc, Schneelocke, Scorpion0422, Scottmsg, SkyLined, Slakr, StanShebs, Stevertigo, StewartMH, Strait, Suslindisambiguator, Susvolans, TUSHANT JHA, TauLibrus, Tavilis, Teles, TenOfAllTrades, TheEditrix2, TimothyRias, Tom Lougheed, Tomvds,TriTertButoxy, TwistOfCain, Twocount, Tyco.skinner, Urvabara, Van helsing, VovanA, Vulcan Hephaestus, Vuvar1, Waperkins, WereSpielChequers, WookieInHeat, Xerxes314, Zojj, 272anonymous edits

Image Sources, Licenses and Contributors 79

Image Sources, Licenses and ContributorsImage:First Gold Beam-Beam Collision Events at RHIC at 100 100 GeV c per beam recorded by STAR.jpg  Source:http://en.wikipedia.org/w/index.php?title=File:First_Gold_Beam-Beam_Collision_Events_at_RHIC_at_100_100_GeV_c_per_beam_recorded_by_STAR.jpg  License: Creative CommonsAttribution-Sharealike 2.0  Contributors: Dbc334, Dmgultekin, Doodledoo, FlickreviewR, HAH, Herald Alberich, Kuaile Long, Odie5533, Romanm, Roomba, Saperaud, Túrelio, Yarnalgo, مانفی,5 anonymous editsImage:Standard Model of Elementary Particles.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Standard_Model_of_Elementary_Particles.svg  License: Creative CommonsAttribution 3.0  Contributors: MissMJImage:Elementary particle interactions.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Elementary_particle_interactions.svg  License: Public domain  Contributors:en:User:TriTertButoxy, User:StanneredImage:Particle chart Log.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Particle_chart_Log.svg  License: Public Domain  Contributors: Arivero, 1 anonymous editsImage:Hqmc-vector.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Hqmc-vector.svg  License: Creative Commons Attribution 3.0  Contributors: VermillionBirdImage:Particle overview.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Particle_overview.svg  License: Creative Commons Attribution-Sharealike 3.0  Contributors: HeadbombFile:CMS Higgs-event.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:CMS_Higgs-event.jpg  License: unknown  Contributors: Lucas TaylorFile:AIP-Sakurai-best.JPG  Source: http://en.wikipedia.org/w/index.php?title=File:AIP-Sakurai-best.JPG  License: Public Domain  Contributors: selfFile:Higgs, Peter (1929).jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Higgs,_Peter_(1929).jpg  License: Creative Commons Attribution-Sharealike 2.0  Contributors: Gert-MartinGreuelFile:One-loop-diagram.svg  Source: http://en.wikipedia.org/w/index.php?title=File:One-loop-diagram.svg  License: GNU Free Documentation License  Contributors: JabberWokImage:Higgs-Boson-March-2011.png  Source: http://en.wikipedia.org/w/index.php?title=File:Higgs-Boson-March-2011.png  License: Creative Commons Zero  Contributors: aesir.leImage:Gluon-top-higgs.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Gluon-top-higgs.svg  License: GNU Free Documentation License  Contributors:http://en.wikipedia.org/wiki/User:JabberWokImage:BosonFusion-Higgs.svg  Source: http://en.wikipedia.org/w/index.php?title=File:BosonFusion-Higgs.svg  License: GNU Free Documentation License  Contributors: derivative work:~ Booya Bazooka BosonFusion-Higgs.png: User:Harp 12:43, 28 March 2007Image:Mecanismo de Higgs PH.png  Source: http://en.wikipedia.org/w/index.php?title=File:Mecanismo_de_Higgs_PH.png  License: GNU Free Documentation License  Contributors:YrithinndImage:LHC.svg  Source: http://en.wikipedia.org/w/index.php?title=File:LHC.svg  License: Creative Commons Attribution-Sharealike 2.5  Contributors: User:HarpFile:BosonFusion-Higgs.svg  Source: http://en.wikipedia.org/w/index.php?title=File:BosonFusion-Higgs.svg  License: GNU Free Documentation License  Contributors: derivative work:~ Booya Bazooka BosonFusion-Higgs.png: User:Harp 12:43, 28 March 2007File:Location Large Hadron Collider.PNG  Source: http://en.wikipedia.org/w/index.php?title=File:Location_Large_Hadron_Collider.PNG  License: Creative Commons Attribution-Sharealike2.0  Contributors: diverse contributors; mashup by User:ZykureFile:LHC quadrupole magnets.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:LHC_quadrupole_magnets.jpg  License: Creative Commons Attribution 2.0  Contributors: Andrius.vFile:Construction of LHC at CERN.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Construction_of_LHC_at_CERN.jpg  License: GNU Free Documentation License Contributors: Andrius.v, Deadstar, Square87File:Higgs, Peter (1929)3.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Higgs,_Peter_(1929)3.jpg  License: Creative Commons Attribution-Sharealike 2.0  Contributors:Gert-Martin GreuelImage:Baryon decuplet.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Baryon_decuplet.svg  License: Public Domain  Contributors: Original uploader was Wierdw123 aten.wikipedia (Original text : Wierdw123 (talk))Image:Quark structure proton.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Quark_structure_proton.svg  License: Creative Commons Attribution-Sharealike 2.5  Contributors:Made by Arpad HorvathImage:Noneto mesônico de spin 0.png  Source: http://en.wikipedia.org/w/index.php?title=File:Noneto_mesônico_de_spin_0.png  License: Public Domain  Contributors: E2m, Pieter Kuiper,StanneredImage:Helium atom QM.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Helium_atom_QM.svg  License: Creative Commons Attribution-ShareAlike 3.0 Unported  Contributors:User:Yzmo

License 80

LicenseCreative Commons Attribution-Share Alike 3.0 Unported//creativecommons.org/licenses/by-sa/3.0/