The Higgs Boson - Cornell Universitymaxim/talks/CIPT_Fall12_Higgs.pdfMaxim Perelstein, LEPP/Cornell U. CIPT Fall Workshop, Ithaca NY, October 27 2012 The Higgs Boson: Why We Needed

Maxim Perelstein, LEPP/Cornell U.CIPT Fall Workshop, Ithaca NY, October 27 2012

The Higgs Boson:Why We Needed It, and

How We Found It

Saturday, October 27, 2012

The basic question of particle physics: What is the world made of? What is the smallest indivisible building block of matter? Is there

such a thing?

In the 20th century, we made tremendous progress in observing

smaller and smaller objects

Today’s accelerators allow us to study matter on length scales as

short as 10^(-19) m


Large Hadron Collider (LHC) at CERN (Geneva, Switzerland)


Particle Collider is a Giant Microscope!

• Optics: diffraction limit,

• Quantum mechanics: particles waves,

• Higher energies shorter distances:

• Nucleus: proton mass

• Colliders before LHC:

• LHC:

!min ! !

! ! h/p

! ! 10!13

cm Mpc2! 1 GeV

E ! 100 GeV ! ! 10!16

cm

E ! 1000 GeV ! 1 TeV ! ! 10!17

cm


Particle Colliders Can Create New Particles!

• All naturally occuring matter consists of particles of just a few types: protons, neutrons, electrons, photons, neutrinos

• Most other known particles are highly unstable (lifetimes << 1 sec) do not occur naturally

• In Special Relativity, energy and momentum are conserved, but mass is not: energy-mass transfer is possible!

• So, a collision of 2 protons moving relativistically can result in production of particles that are much heavier than the protons, “made out of” their kinetic energy

• This is how most elementary particles are discovered!

E = mc2


Another basic question: how did the Universe begin?

High-energy particle collisions, today seen only in accelerators, were quite common in the high-temperature, high-density universe within the first second after the Big Bang!


All our knowledge about subatomic physics is summarized in the Standard Model - the most successful Physics theory ever!

[from: particleadventure.org]Saturday, October 27, 2012

16 different elementary particles have been observed in collider

experiments: 12 “matter particles” and 4 “force particles”

Matter particles are further divided into leptons and quarks

There are 6 leptons and 6 quarks:3 “generations”, 2 leptons and 2

quarks in each

Particles in each row (e.g. u, c and t quarks) are identical except for their masses: t is heavier than c,

which is heavier than u

(The Periodic Table - just like Chemistry, but much simpler and way cooler!)


Some Basic Properties of Matter Particles:

• Each matter particle has an antiparticle, with exactly the same mass but opposite electric charge

• Quarks do not exist as free objects, but only as constituents of “baryons” (a bound state of 3 quarks) and “mesons” (a bound state of a quark and an antiquark)

• Examples:

• 100’s of baryons and mesons have been observed, all can be understood as bound states of the known quarks

• Most particles are unstable (decay into other particles, with lifetimes <<1 sec)

• Exceptions: electron, 3 neutrinos, 2 baryons: proton (uud) and neutron (udd), and their antiparticles, are STABLE

p = uud, n = udd, p = uud, !+= ud, . . .


FOUR FUNDAMENTAL FORCES:Gravitational force: motion of planets, rockets, apples, ...

Electromagnetic force: electricity, radiowaves, light, ...

Weak force: origin of radioactivity

Strong force: binds protons and neutrons in the nucleus

Gravity is very weak – a small magnet can balance the gravitational effect of the entire Earth, BUT:

Only one type of gravitational charge (always attractive), forces add up – very relevant over long distances (while E&M charges cancel)


Modern Picture of Forces: forces between “matter particles” are due to exchange of “force particles”

For example: Electromagnetic force between electrons is due to a photon exchange


Electric and magnetic forces are described byemission and absorption of photon – a particle of zero mass, the force carrier of electromagnetism

Feynman

“Quantum Electrodynamics” - combines Maxwell’s theory of electromagnetism, Special Relativity and Quantum Mechanics

“Feynman Diagram”

[Nobel prize 1965, with Schwinger, Tomonaga]


Weak InteractionsWeak force is described by a theory just like electrodynamics, but instead of photon, mediatingparticles are the W and Z bosons

Feynman diagram for the neutron beta-decay

● Weak force is short-range, with range about cm:

● This implies that the W and Z bosons are massive:

● Discovery of W and Z: CERN, 1983

Vweak !

e!r/r0

r

10!15

M !

h

cr0

! 100 GeV

[Nobel prize 1984: Rubbia, van der Meer]


EM-Weak Unification

10-7

10-6

10-5

10-4

10-3

10-2

10-1

1

10

103

104

HERA I high Q2 e-p

H1 e-p CCZEUS e-p CC 98-99SM e-p CC (CTEQ5D)

H1 e-p NCZEUS e-p NC 98-99SM e-p NC (CTEQ5D)

y < 0.9

Q2 (GeV2)

d!/d

Q2 (p

b/G

eV2 ) • EM and Weak forces

become equally strong at short distances of order cm

• Same theory describes both forces in a unified framework

10!15

[blue=EM, red=weak][Nobel prize 1979: Glashow,

Salam, Weinberg]


Strong InteractionsStrong force is also described by a theory very similar to electrodynamics, the force particle is the gluon

Due to peculiar nature of the gluon, the strong force grows with distance between charges: V ! r

Only quarks experience the strong force, leptons are immune to it (neutral). This explains why quarks are confined and leptons are not!

At short distances, the strong force gets weaker - the closer together you bring the quarks, the more freedom they feel! (”asymptotic freedom”)

[Nobel prize 2004: Gross, Politzer, Wilczek]


Gravitational InteractionsGravitational force is supposedly described by a theory very similar to electrodynamics, the force particle is the graviton

Just like photon is a quantum of electromagnetic wave, graviton would be a quantum of gravitational wave

Gravitaional waves are predicted by General Relativity, and their indirect effects have been seen, but NO direct observation so far!

LIGO gravitational wave detectors in Hanford, Washington and Livingston, Louisiana


This concludes our brief tour of matter particles...

and their interactions/force particles!


Predictive Power of the Standard Model

• The Standard Model is not just a list of particles and a classification - it is a theory that makes detailed, precise quantitative predictions!

• Consider a head-on collision of a 100 GeV electron and a 100 GeV antielectron (”positron”). Possible outcomes:

• Quantum mechanics: there is no way to know for sure which outcome will occur in a given collision, but the SM predicts probabilities (”cross sections”) of each outcome, plus details like directions of the produced particles, etc.

• Works spectacularly well! (some predictions experimentally verified to 0.1% accuracy)

e+e!, µ+µ!, !+!!, pp, W+W!, e+e+e!e!, . . .


Symmetry in the Standard Model

• Mathematical consistency of the Standard Model relies on a set of symmetries: relations among various particles (e.g. up and down quark).

• If symmetries are exact, predictions are incorrect: for example, all matter and force particles are predicted to be massless


Spontaneous Symmetry Breaking• Solution: Symmetries are Broken, but

very subtly: “Spontaneously”

• Analogy: Empty space is isotropic, but space inside a capacitor is not - E field breaks the symmetry!

• In the SM, the Universe is assumed to be filled with a field: Higgs field

• Higgs field is scalar (like e.g. temperature) space still isotropic

• Higgs field breaks symmetries between particles gives them mass! HiggsEnglert Brout

1964Saturday, October 27, 2012

Mass Generation, in Cartoons• Mass is due to “bumping”

into the Higgs field

• Different particles have different masses due to different strength of their interaction with the Higgs field (”charge”)


Mass Generation, in Cartoons• Mass is due to “bumping”

into the Higgs field



Particles and Fields• Field Wave Particle

• Electric+Magnetic Fields EM Waves Photons

• Relativity and Quantum Mechanics guarantee: particle field! e.g. electron field, proton field, ..., Higgs particle!


http://www.google.com/imgres?imgurl=http://1.bp.blogspot.com/-wcViWyagb3M/Tckrch5LG5I/AAAAAAAAAD4/2FZqvSyaKkQ/s1600/The%2BElectromagnetic%2BSpectrum.jpg&imgrefurl=http://frazerphysics.blogspot.com/2011_05_01_archive.html&h=346&w=500&sz=29&tbnid=8K533ZWwFJMHUM:&tbnh=83&tbnw=120&prev=/search%3Fq%3DEM%2Bwaves%26tbm%3Disch%26tbo%3Du&zoom=1&q=EM+waves&usg=__biHZv_qfYRnsBJcANaP-xMGplwI=&docid=DR7F7fE975zXUM&hl=en&sa=X&ei=AJKFUI_pM8-x0QHnsIDYAw&ved=0CEcQ9QEwAw&dur=146


http://www.google.com/imgres?imgurl=http://www.ygoy.com/wp-content/uploads/2009/10/em_spectrum.jpg&imgrefurl=http://ygoy.com/2009/10/13/do-mobile-phone-radiations-affect-the-human-body/&h=389&w=619&sz=29&tbnid=g9mJODrVrQ-cOM:&tbnh=76&tbnw=121&prev=/search%3Fq%3DEM%2Bwaves%26tbm%3Disch%26tbo%3Du&zoom=1&q=EM+waves&usg=__Wo-g8DXAl8LSXnu5jjaHbvh5MHc=&docid=zwy08uDCuF8iYM&hl=en&sa=X&ei=AJKFUI_pM8-x0QHnsIDYAw&ved=0CEoQ9QEwBA&dur=1086





Higgs: SM Predictions• To recap: Massive particles + symmetries Higgs field Higgs

particle - the 17th pillar of the SM!

• Predictions of the SM:

• Mass: roughly between 100* and 1000 GeV

• “Cross Section”: How many produced in a given number of proton-proton collisions (~100,000/year at the LHC, compared to 10^16 total collisions)

• “Lifetime”: about 10^(-22) sec

• Decay “channels”: 2 photons, 2 W’s, 2Z’s, 2 quarks, etc., and relative rates for each one (“branching fractions”)


Finding the Higgs• Accelerate 2 protons to very high energies

• Collide them at “interaction points”: energy mass

• Produced Higgs bosons instantly decay, for example into two photons

• Surround interaction points by detectors which can detect Higgs decay products

• Analyze the data from the detectors: find a way to distinguish the “Higgs event” from 10^11 non-Higgs events


LHC Animation: “The Bottle to Bang”

http://www.youtube.com/watch?v=HDToMXog1gEText

http://cdsweb.cern.ch/record/1125472

or






Detectors at the LHC

“Compact Muon Solenoid” (CMS)


Detectors at the LHC

ATLAS Detector


Anatomy of the CMS


Anatomy of the CMS


Higgs Candidate Event 1

2 photons


Higgs Candidate Event 2

2 electrons + 2 positrons


Discovery!

“Background”(non-H events)

“Signal”(Higgs events)

[CERN Seminar, July 4 2012]


CERN Seminar, July 4, 2012

38 years after the theoretical prediction...


Future: Is This Really the SM Higgs?

• Mass is due to “bumping” into the Higgs field


• Reflected in relative rates of various decay channels: 2 photon, WW, 2 quarks, etc.


Future: Is This Really the SM Higgs?

• Mass is due to “bumping” into the Higgs field


• Reflected in relative rates of various decay channels: 2 photon, WW, 2 quarks, etc.

Want to Measure These Rates

Experimentally!


The Next Collider: ILC?

Superconducting RF Cavities, developed at Cornell

ILC = “International

Linear Collider”

a.k.a. “Higgs Factory”:

1000000’s of H’s in clean

environment


Conclusions/Recap• Standard Model of particle physics is an enormously successful

theory of physics at distances down to ~10^(-18) cm

• Many predictions of the model verified experimentally over the last ~40 years, some with great precision

• The Higgs field permeating the Universe is necessary for the model to work, but not experimentally confirmed yet

• Search for the Higgs is a major focus for experiments at the Large Hadron Collider at CERN

• Particle discovered at CERN looks roughly consistent with the Higgs, but more work needed to test/confirm this hypothesis


Documents

The Higgs Boson - Cornell Universitymaxim/talks/CIPT_Fall12_Higgs.pdfMaxim Perelstein, LEPP/Cornell U. CIPT Fall Workshop, Ithaca NY, October 27 2012 The Higgs Boson: Why We Needed