Overview The quest of Particle Physics research is to understand the fundamental particles of nature and their interactions.

Our understanding is about to take a giant leap ….. the Large Hadron Collider has started.

These lectures are aimed to explain the background to our current understanding and the challenges involved in bringing the LHC to fruition …. along with the fruits of the first data…

I hope you enjoy them !

Lecture 1 : Matter and Forces Lecture 2 : Electroweak Unification Lecture 3 : Beyond the Frontier ….. The LHC accelerator Lecture 4 : The LHC Experiments Lecture 5 : First Results and Expectations 2

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Nucleus and electrons bound by electromagnetic force. 10-10 m

Photon

The Atom

… nucleus is now 3 mm across !

Atoms are 99.9999999999999% empty space

… blow atom up to size of Millennium Dome...

10-14 m Protons and neutrons in nucleus bound by strong nuclear force.

Gluon

Quarks bound by the strong force.

<10-15 m

Nucleus and electrons bound by electromagnetic force. 10-10 m

Photon

The Atom

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ATOM Electrons bound to atom by electromagnetic force

NUCLEUS Nuclei held together by strong “nuclear” force

NUCLEON Protons and neutrons held together by the strong force

Binding energy 10 GeV

Binding energy 10 eV

γ e-

Binding energy 0.1 MeV π

1fm = 10-15m

Size: Atom ~10-10 m, e- < 10-18m Charge: Atom is neutral, electron –e Mass: Atom mass ~ in nucleus, me= 0.511 MeV/c2

Chemical properties depend on Z.

Size: Nucleus (medium A) ~ 5fm

Size: p, n ~ 1fm Charge: p +e n 0 Mass : p, n = 939.57 MeV/c2 ~ 1836 me

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Energy and Mass Common practise in Particle Physics NOT to use SI units.

  Energies are measured in units of eV:

1 eV = Energy an electron acquires when it is accelerated through a potential difference of 1V.

Common to use: KeV (103 eV) MeV (106 eV) GeV (109 eV) TeV (1012 eV)

  Masses quoted in units of MeV/c2 or GeV/c2

e.g. Electron mass

€

me = 9.11×10−31 kg = 9.11×10−31( ) 3×108( )2 1.602 ×10−19

= 5.11×105 eV c 2 = 0.511 MeV c 2

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Matter We now know that all matter is made of two types of elementary particles (spin ½ fermions):

LEPTONS: e.g. e-, νe

QUARKS: e.g. up quark (u) , down quark (d) proton (uud)

And….. Antimatter……

Electron e-

Quark u

Proton uud

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Antimatter

Every matter particle has a partner which has exactly the same properties except a charge which is opposite in sign.

Electron e- Positron e+ Quark u Antiquark

Proton uud Antiproton

Exploit matter-antimatter annihilation to produce new particles. Energy can be transformed back into any type of mass.

e+ e- E=mc2

u

uud

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Matter: 1st Generation Almost all phenomena you will have encountered can be described by the interactions of FOUR spin ½ particles:

THE FIRST GENERATION

The proton and neutron are the lowest energy states of the combination of 3 quarks:

Particle Symbol Type Charge Units of e

Electron e- Lepton -1 Neutrino νe Lepton 0 Up Quark u Quark +2/3 Down Quark d Quark -1/3

u u d

d d u p n

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Matter: 3 Generations Nature is not quite so simple. There are THREE generations of fundamental fermions:

  Each generation e.g. (µ-, νµ, c, s) is an exact copy of (e-, νe, u, d)   The only difference is the mass of the particles: the 1st generation are the lightest and the 3rd generation are heaviest.   Clear symmetry – origin of 3 generations is NOT UNDERSTOOD.

1st Generation 2nd Generation 3rd Generation Electron e- Muon µ- Tau τ- Electron Neutrino

νe Muon Neutrino

νµ Tau Neutrino

ντ

Up quark u Charm quark

c Top quark t

Down quark d Strange quark

s Bottom quark

b

3 Generations

Up u

Down d Electron e

Neutrino νe

Charm c

Strange s Muon µ

Neutrino νµ

Top t

Bottom b Tau τ

Neutrino ντ

All normal matter consists of u and d quarks and electrons

There are two other families of quarks and leptons, which are heavier

The heavier particles decay to lighter ones

Quarks Leptons

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Quarks Quarks experience ALL the forces (electromagnetic, strong, weak)   Spin ½ fermions   Fractional charge   6 distinct flavours   Quarks come in 3 colours Red, Green, Blue   Quarks are confined within HADRONS e.g.

COLOUR is a label for the charge of the strong interaction. Unlike the electric charge of an electron (-e), the strong charge comes in 3 orthogonal colours RGB.

Gen.

Flavour Charge (e) Approx. Mass (GeV/c2)

1st u +2/3 0.35

d -1/3 0.35

2nd c +2/3 1.5

s -1/3 0.5

3rd t +2/3 171

b -1/3 4.5 u u d u d

+antiquarks u,d,…

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Leptons Particles which DO NO INTERACT via the STRONG interaction.   Spin ½ fermions   6 distinct FLAVOURS   3 charged leptons: e-, µ-, τ-

µ and τ unstable

  3 neutral leptons: νe, νµ, ντ Neutrinos are stable and (almost?) massless νe mass < 3 eV/c2

νµ mass < 0.17 MeV/c2

ντ mass < 18.2 MeV/c2 +antimatter partners, e+, νe

  Charged leptons only experience the electromagnetic and weak forces

  Neutrinos only experience the weak force

Gen.

Flavour Charge (e) Approx. Mass (MeV/c2)

1st e- -1 0.511

νe 0 Massless ?

2nd µ- -1 105.7

νµ 0 Massless ?

3rd τ- -1 1777.0

ντ 0 Massless ?

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Hadrons   Single free quarks are NEVER observed, but are always CONFINED in bound states, called HADRONS.   Macroscopically hadrons behave as point-like COMPOSITE particles.

Hadrons are of two types:

MESONS (qq) Bound states of a QUARK and an ANTIQUARK All have INTEGER spin 0, 1, 2,… Bosons e.g. charge = +2/3e + 1/3e = +1e charge = -2/3e – 1/3e = -1e

BARYONS (qqq) Bound states of 3 QUARKS All have HALF-INTEGER spin 1/2, 3/2,… Fermions e.g.

PLUS ANTIBARYONS e.g.

q q

q q q €

π + ≡ ud( )

€

p ≡ uud( )

€

n ≡ udd( )€

π− ≡ ud( )

€

(qqq)

€

p ≡ uud( )

€

n ≡ udd( )

Periodic Table of the Elements

Differences between materials are due simply to the number of protons and electrons in their atoms.

Only three elements are formed in the Big Bang

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Forces

Classical Picture: A force is “something” which pushes matter around and causes objects to change their motion (Newtons II).

e.g. Electromagnetic forces arise via the action at a distance of the electric and magnetic fields.

Newton: ”… that a body can act upon another at a distance, through a vacuum, without the mediation of anything else,…, is to me a great absurdity”

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Forces

Quantum Mechanically: Forces arise due to exchange of VIRTUAL FIELD QUANTA (Gauge Bosons): “second quantization”.

Field strength at any point is uncertain

Number of quanta emitted and absorbed

Massless particle e.g. photon

Strong

Polonium

e-

Astatine

Weak

Electromagnetism Gravity

Forces

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Gauge Bosons GAUGE BOSONS mediate the fundamental forces   Spin 1 particles (i.e. Vector Bosons)   No generations   The manner in which the Gauge Bosons interact with the

leptons and quarks determines the nature of the fundamental forces.

Force Boson Spin Strength Mass (GeV/c2)

Strong Gluon g 1 1 Massless

Electromagnetic Photon γ 1 10-2 Massless

Weak W and Z W±, Z0 1 10-7 80, 91

Gravity Graviton ? 2 10-39 Massless

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Range of Forces The range of a force is directly related to the mass of the exchanged bosons.

Due to quark confinement, nucleons start to experience the strong interaction at ~ 2 fm

€

Range ~ 1mass

Force Range (m) Strong Strong (Nuclear)

∞

10-15

Electromagnetic ∞

Weak 10-18

Gravity ∞ 10

-26

10-23

10-20

10-17

10-14

10-11

10-8

10-5

10-2

10

104

107

1010

10-4

10-3

10-2

10-1

1 10 102

103

Gravitational

Force

Electromagnetic

Force

Weak Force

Strong Force

(quarks)

Strong Force

(hadrons)

Distance (fm)

Forc

e (G

eV/f

m)

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The Standard Model Spin ½ Fermions

LEPTONS

QUARKS

PLUS antileptons and antiquarks.

Spin 1 Bosons Mass (GeV/c2) Gluon g 0 STRONG Photon γ 0 EM W and Z Bosons W±, Z0 91.2/80.3 WEAK

The Standard Model also predicts the existence of a spin 0 HIGGS BOSON

which gives all particles their masses via its interactions.

Charge (units of e) -1 0

2/3 -1/3

Explains all the data we have so far…….

but there are many unanswered questions...

The Standard Model of Particle Physics

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Theoretical Framework Macroscopic Microscopic Slow Classical Mechanics Quantum Mechanics

Fast Special Relativity Quantum Field Theory

The Standard Model is a collection of related QUANTUM FIELD THEORIES that describe particle interactions.

ELECTROMAGNETISM: QUANTUM ELECTRODYNAMICS (QED) 1948 Feynman, Schwinger, Tomonaga (1965 Nobel Prize)

ELECTROMAGNETISM: ELECTROWEAK UNIFICATION +WEAK 1968 Glashow, Weinberg, Salam (1979 Nobel Prize)

STRONG: QUANTUM CHROMODYNAMICS (QCD) 1974 Politzer, Wilczek, Gross (2004 Nobel Prize)

Feynman Diagrams Richard Feynman devised a pictorial method for calculating the interaction between fundamental particles

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Electron-proton scattering

ELECTROMAGNETIC e- e-

p p Quark-antiquark annihilation

q q STRONG

u d

d

u d

u

e-

n p

€

gW

Neutron decay WEAK

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γ

Summary of Standard Model Vertices ELECTROMAGNETIC STRONG WEAK CC WEAK NC

(QED) (QCD)

γ

g

W-

W- Z0

Z0

+antiparticles

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QED Quantum Electrodynamics is the theory of the electromagnetic interaction

Some QED processes :

Compton Scattering

Pair production

e+e- Annihilation

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γ γ

e- e- e-

Nucleus

€

γ e- →γ e-

€

γ → e+e-

1

10

102

103

104

105

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5

Angle (radians)

dσdΩ

SLAC e- scattering (1972)

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Discovery of Quarks Virtual γ carries energy and momentum

Large momentum ⇒ small wavelength Large energy ⇒ high frequency

Photon oscillates rapidly in space and time ⇒ probes short distances and short time.

Small E,p

E, p increases

Large E,p

Rutherford Scattering

Excited states

Elastic scattering from quarks in proton λ<< size of proton

E = 8 GeV

Expected Rutherford scattering

e- e-

p p

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SLAC 1970

Rutherford 1911

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Experimental Tests of QED QED is an extremely successful theory tested to very high precision.

Example:   Magnetic moments of e±, µ± :

  For a point-like spin ½ particle: Dirac Equation

However, higher order terms introduce an anomalous magnetic moment i.e. g not quite 2.

v

O(α)

v

O(α4) 12672 diagrams

O(1)

• • • •

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O(α3)

  Agreement at the level of 1 in 108.   QED provides a remarkable precise description of the

electromagnetic interaction ! €

ge − 22

= 11596521.869 ± 0.041( ) ×10−10

ge − 22

= 11596521.3± 0.3( ) ×10−10Experiment

Theory

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QCD QED: is the quantum theory of the electromagnetic interaction.

  mediated by massless photons   photon couples to electric charge   strength of interaction:

QCD: is the quantum theory of the strong interaction.

  mediated by massless gluons   gluon couples to “strong” charge, called “COLOUR”   only quarks have non-zero “COLOUR”, therefore only quarks feel the strong interaction. Quarks carry “COLOUR” (red, green, blue) Antiquarks carry “ANTI-COLOUR” (anti-red, anti-green, anti-blue)

  strength of interaction:

€

α = g2 = e2 ~ 1137

€

α s = gs2 ~ 1

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Gluons QCD looks like a stronger version of QED. However, there is one BIG difference and that is GLUONS also carry “COLOUR”.

⇒ GLUONS CAN INTERACT WITH OTHER GLUONS

3 GLUON VERTEX 4 GLUON VERTEX Example: Gluon-gluon scattering gg → gg

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Confinement NEVER OBSERVE single FREE quarks or gluons.   Quarks are always confined within hadrons   This is a consequence of the strong interaction of gluons.

Qualitatively, compare QCD with QED:

Self interactions of the gluons squeeze the lines of force into a narrow tube or STRING. The string has a “tension” and as the quarks separate the string stores potential energy.

Energy stored per unit length in field ~ constant

Energy required to separate two quarks is infinite. Quarks always come in combinations with zero net colour charge ⇒ CONFINEMENT.

QCD Colour field

QED Electric field

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Jets Consider the quark, anti-quark pair produced in e+e- annihilation:

As the quarks separate, the energy in the colour field (“string”) starts to increase linearly with separation. When the energy stored exceeds 2mq, new quark, anti-quark pairs can be created.

As energy decreases… hadrons (mainly mesons) freeze out

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As quarks separate, more quark, anti-quark pairs are produced. This process is called HADRONIZATION. Start out with quarks and end up with narrowly collimated JETS of HADRONS.

Typical event

The hadrons in a quark (antiquark) jet follow the direction of the original quark (antiquark).

Consequently, is observed as a pair of back-to-back jets.

JET

JET

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Evidence for Gluons In QED, electrons can radiate photons. In QCD, quarks can radiate gluons.

Example:

In QED we can detect the photons. In QCD, we never see free gluons due to confinement.

Experimentally, detect gluons as an additional jet: 3-JET events.

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LEP Event (1990)

Discovery of gluons (1978)

Summary So far we have discussed

  the fundamental particles (quarks and leptons)   the forces of nature (electromagnetism, strong, weak and gravity)   the force carriers (photon, gluon, W and Z and the graviton)   the theory of electromagnetism (Quantum Electrodynamics)   the theory of the strong interaction (Quantum Chromodynamics)

Next lecture we will discuss

The Weak Force and Electroweak Unification

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