12948957 introduction to particle physics

8/8/2019 12948957 Introduction to Particle Physics

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PHY-653 EPP Introduction to Particle Physics Slide 2 of 23

Particle Hierarchy

Quarks and Electrons are made of???Quarks and Electrons are"Elementary Particles "

Everyday Objects are made ofMolecules.

Moleculesare made ofAtoms.Atoms are made ofNuclei and Electrons.Nuclei are made ofProtons and Neutrons.Protons and Neutrons are made ofQuarks.


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Elementary Particles

Quarks have electric charge -e or +eQuarks come in six ' flavours' :

up (u), down(d), strange (s), charm (c), bottom (b), top (t)Each quark comes in 3 ' colours' - Red, Green, Blue

Elementary Particles are 'point like' fundamental spin fermions(obey Fermi-Dirac Statistics). They have no discernible size or

structure. There are two types Quarks and Leptons:

Leptons - have electric charge 0 or eLeptons also come in six flavours :

electron , muon , tau ,electron neutrino , muon neutrino , tau neutrino

Leptons don't have colour


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Antiparticles

All quarks and leptons have antiparticle partners, with allquantum numbers reversed, but with the same masses.

Antileptons: , , , , ,

Antiquarks: , , , , ,

Antiquarks have anticolour.

Put a bar over the symbolCharge + rather than -

The anti-electron is known as the positron.

Note: Electric Charge has values +, - , 0Colour Chargehas values r, g, b, , ,

When particle and antiparticle meet theyannihilate to give energy with all quantumnumbers zero (usually as photons).


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Particle Families

(tau neutrino)(tau neutrino)(tau neutrino)(tau neutrino)

(muon neutrino)(muon neutrino)(muon neutrino)(muon neutrino)

(electron neutrino)(electron neutrino)(electron neutrino)(electron neutrino)

0000

(tau)(tau)(tau)(tau)

(muon)(muon)(muon)(muon)

(electron)(electron)(electron)(electron)----eeee

LeptonsLeptonsLeptonsLeptons

bbbb(bottom)(bottom)(bottom)(bottom)

ssss(strange)(strange)(strange)(strange)

dddd(down)(down)(down)(down)

----eeee

tttt(top)(top)(top)(top)

cccc(charm)(charm)(charm)(charm)

uuuu(up)(up)(up)(up)

++++eeee

QuarksQuarksQuarksQuarks

ThirdThirdThirdThirdSecondSecondSecondSecondFirstFirstFirstFirstGenerationGenerationGenerationGeneration

Quarks and Leptons can be arranged in 3 ' families' or ' generations ':


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Hadrons

Although leptons (e.g. electrons) exist as free particles, freequarks have never been observed. They always form bound(colourless) states called Hadrons. There are two types ofhadrons.

Baryons consist of 3 quarks (Antibaryons 3 antiquarks)e.g. proton uud antiproton

neutron udd antineutron

Mesons (antimesons) consist of a quark and an antiquark

e.g. pion u antipion dneutral pion uor d

NO other combinations e.g. qq or qqqq observed

e +e +(-e) = e

e +(-e) +(-e) = 0

e +e = e


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Forces

The particles (Quarks and Leptons) interact through different' forces ' which we understand as due to the exchange of ' fieldquanta' known as ' gauge bosons'.

Electromagnetism (QED) photon () exchangeStrong Interaction (QCD) gluon(g) exchangeWeak Interaction W and Z exchangeGravity Graviton exchange (?)

There is also postulated an, as yet undiscovered (?) scalar ( = spin zero) particle called the Higgs boson ( ) which is needed to explain why the W and Z have mass (i.e. aren't massless).

The so called Standard Model of Particle Physics is based on the

six quarks and six leptons interacting via these gauge bosons.

The photon, gluon, W and Z are spin 1, the graviton spin 2.


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Exchange Forces

By exchanging

the ball, the skaters are forced apart.

If you didn't see the ball you would think there

was a repulsive force between them.


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Range of Forces

The range of the interaction is related to the mass of the exchangeparticle M.

x = c / M

The photon has zero mass infinite range

The W has a mass of ~80 GeV/ 197 MeV fm / 80103 2 10-3fm

The maximum distance the exchange particle can travel in this time isx = c t

(c is the maximum velocity it can have)

c in funny units (see later)

Converts GeV to MeV

x = c / E = c / M

An amount of energy E = M is 'borrowed' for a time tgoverned by the Uncertainty Principle E t ~ i.e. t = / E.


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Forces

The weak force acts between all quarks and leptons.

NeutralLeptons

ChargedLeptons

Quarks

StrongEMWeak

The strong force acts between all quarks.

The electromagnetic force acts between all charged particles.


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Feynman Diagrams

Feynman Diagramsare like circuit diagrams they show whatis connected to what but not the detailed momentum vectors lengths and angles are not relevant.

Conventions:

A particle movingforward in timeand space

A particle moving(~instantaneously) fromone point to another

A particle at rest


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Annihilation Diagrams

Annihilation/Formation Diagram. Particles A and B collideto form particle X which later decays to C and D.

At each vertex , electric charge must be conserved and, except in Weak Interactions, quark or lepton flavours.


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Virtual ParticlesIn both previous cases particle X is ' virtual' and the time itexists is governed by the uncertainty principle E t ~ . Themass of particle X is usually not its rest mass.

If and electron and positron annihilate, X is a photon ( ) with zero charge, zero momentum and energy 2E e and hence an apparent mass of 2E e / .

If two electrons scatter, X is a photon ( ) with zero charge, momentum 2p e and zero energy and hence an apparent imaginary mass of -p e 2 /c 2 .

E2 = p2c2 +m2c4


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Electromagnetism

Photons mediate the force between protons and electrons.

At a particle physics level the interaction is with the quarks


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Strong Interaction (1)

Gluons hold the proton and neutron together and are responsible for the Strong force between them.


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Strong Interaction (2) +p n +

d + uududd + d

Virtual exchange

Quarklines arecontinuous


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Weak Interaction (1)

Beta decay n p + +

Mediated by charged W exchange


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Weak Interaction (2)

Mediated by neutral

Z exchange

Neutrino scattering

off an electron


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Use of Feynman Diagrams

Although they are used pictorially to show what is going on,Feynman Diagrams are used more seriously to calculate crosssections or decay rates .

Free particle Vertex~charge

Propagator ~ 1p2 + m2

Square the amplitude to get the intensity/probability(cross section or decay rate).

Addthe amplitudes for each diagram (including interference).

Calculatethe amplitude by multiplying together.

Assignvalues to each part of the diagram:

Drawall possible Feynman Diagrams for the process :


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Particle Physics and CosmologyIncreasing connection between Particle Physics and Cosmologythe more fundamental the particle, the earlier they were createdduring the early Universe ( Big Bang).

Today's particle accelerators probe further and further back intime to moment of Big Bang:

At LEP Accelerator energy ~ 90 GeV90109 eV / 8.6 10 -5 eV K-11015KEquivalent to < 10-12s after Big Bang

where k is Boltzmann's constant ( 8.6 10-5 eV K-1)

Energy Temperature

E kT


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Probing the Early Universe

www.counterbalance.net

Particle Physics today probes this region


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Understanding the Universe

ParticlePhysics

Composition

InitialConditions

Formation ofStructure

Understanding ofParticle Physics

Understanding ofUniverse at Big Bang

Understanding ofUniverse today

H, ,

Matter/Antimatter,Dark Matter,

Nucleosynthesis

Cosmic MicrowaveBackground, Large ScaleStructure

12948957 introduction to particle physics

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