Particle Physics

Strong

Gluons

Found by 'Fat Jets' as opposed to normal jets

Interacts with Colour

Has Colour itself, so is self interacting

Colour is normally a Colour-Anticolour pair, i.e. Red-Antigreen

Weak

W and Z Bosons with approximately 80 and 90 GeV of Mass respectively

W Changes Flavour of quark

Z can be made in high energy annihilation, can decay to anything neutral

Interact with all particles

Very Short Range

Electromagnetic

Photons

Interact with charge

Photons are chargeless and massless

We Only consider L=0, so J = S. Assume only quarks are up down and strange

For baryons you get a 1/2+ Octet and a 3/2+ Decuplet

For Mesons you get a 0- Nonet and a 1- Nonet

If we allow all quarks and L > 0 then it gets very complicated

Old

Old Laws are always conserved

Energy

Momentum

Charge

New

These are not necessarily always conserved

Quantum Numbers such as: B, S, I, C, L

Parity

This transfers co-ordinates of a system

Normal Vectors change sign

e.g. r -> -r

e.g. v -> -v

e.g. p -> -p

Axial Vectors are unchanged

e.g. L = r x p -> -r x -p = L

Parity is a multiplicative quantum number

Particles have intrinsic parity of +/- 1

Parity is conserved in Strong and EM interactions, but not Weak

Charge Conjugation

Changes the charge of a particle

Particles which are their own antiparticles, C^2 returns initial particle

Charge Conjugation is a mutliplicative quantum number

It is conserved in Strong and EM, but not Weak

CP

The action of both the Charge Conjugation and the Parity transformation on a particle

This is conserved for weak (In almost all circumstances)

This implies that particles and antiparticles have similar properties, quantum numbers aside

e.g. Mass should be the same

e.g. Spin should be the same etc.

A Resonance is a particle that decays via the storng interaction

It does not violate energy or quantum number violations

Can be found by Formation or Production

Formation

Fire particles into a target at varying energies

Look for outcoming particles and measure their Centre of Mass Energy

Will get a significant spike at the mass of the resonance particle due to its creation only happening when there is momentum conservation

Used by Richter to find what he called the Psi Particle

Richter is Good

Production

Brute Force method, by plowing your beam into target with loads of energy

Measure the Invariant Mass of all produced particles, should get a spike at resonance mass above the average mass of any 2 particles

Used by Ting to find what he called the J Particle

Ting is Bad

Zweig Suppression

A Process is Zweig Suppressed if it requires many gluons to do it

Even if the process with less gluons has a smaller Q value

Because each vertex of a feynman diagram adds a power to the coupling constant, lowering brancing ratio

Crossection is proportional to Coupling Constant ^ n for n/2 gluons

Cabibo Suppression

Not needed for this course

Dictates that quarks don't tend to decay from the top-generation to the bottom-generation.

J/Psi Particle

Consists of c cbar, so has 'hidden charm'

Was first evidence for something beyond 3 quark model

Longer than usual strong decay lifetime due to Zweig suppressed decay

Detected independantly by Richter and Ting

Upsilon Particle

Consists of b bbar, quark model now has 5 base quarks

Top Quark

Predicted because something had to go with the b quark

Mass was predicted at LEP using virtual t tbar loop

Detected in 1994 by CDF at Tevatron

Doesn't form hadrons, decays too fast

Mass of 172 GeV

Evidence for Quarks

Cross sections

Pi+ p collisions have a cross section of 26 mb, and a ratio of 2 quarks hitting 3 quarks

p p collisions have a cross section of 40 mb, and a ratio of 3 quarks to 3 quarks

The ratio of the cross sections is 2:3

Scattering

Scattering of nucleus by various sources and interactions all found that they scattered off particles of fractional charge

Colour

6 Colours, R, G, B and anti colours

Force Carrier is the Gluon

Gluon has colour-anticolour

Confinement

Becase Gluons can self interact, the further out a quark is, the strong it is attrated to its neighbour

Asymptotic Freedom

The closer 2 quarks are to each other, the less they are bound by gluons

Evidence for Colour

Allows hadrons like sss

Particle decay rates are 3x faster than expected for quarks

Ratio of cross section of hadrons to cross section of muons in e+e- annihilation is 3x larger than expected

Describes direction of spin angular momentum relative to motion

Spin parallel to motion is Right Handed

Neutrinos are always left handed (due to effectively moving at the speed of light)

Antineutrinos are always right handed

M^2 = (Sum of all E)^2 - (Sum of all P)^2

Use this for any kind of collision so that we can set the momentum to zero

Invariant Mass doesn't change in any inertial frame

Comes from the -ve part of E^2 = m^2 + p^2

Can be thought of as a positive energy particle going backwards in time

Has all the properties of normal particles, with all arithmetically additive quantum numbers

Every particle has an antiparticle

Bosons should be massless, but they are not (W & Z)

Analogous to the Muddy Field

Or the interesting Lecturer!

Higgs Bosons has mass, and is being looked for by detectors

C and P are individually violated by trying to perform the actions on neutrinos

But the combined CP action is not violated

It is however violated in, for example, a K0 beam

There is mixing, so that what starts of as pure K0 ends up as a mixture of both K0 and anti K0. This violates CP (handwavey, I know)

In a given reaction for which mass is irrelevant, e, mu and tau all act in the same way. Same applies for neutrinos

In a given decay process of W or Z, quarks are 3 times more like to be produced than leptons due to colour

Determined by LEP by looking at cross sections of Z0 decay

Assume different amounts of flavours and neutrinos and assume lepton universality. Then see which line fits best (see lectures for details and a nice picture)

3 Generations of Particle

Lets look at decay of the X particle

X -> ubar u bar OR d e-

Xbar -> u u OR dbar e+

If there is CP violation, then one process can be more favourable than the other

So Assumed d e- > ubar ubar

Then also u u > dbar e+

If this happens for 1 quark per 1 billion, then explains the matter dominated universe

However, CP violation effects are too small to account for this entirely at the present time.

Combining Strong, EM and Weak Interactions into one Force

Predicted to produce X and Y bosons

X has charge -4/3

Y has charge -1/3

Every particle has a supersymmetric other half

quarks go to squarks

leptons go to sleptons

bosons go to bosinos

SUSY fixes two problems with GUTS

All the forces meet at the same point in SUSY, unlike in GUTS

Lifetime of the proton is extended to a just acceptable level of 10^33 years