1Phy107 Lecture 411Today: Review Particle Physics• Particles and fields: a new picture• Quarks and leptons• The weak interaction• Unification and mass• The approach of string theory• Particle physics and the universeExam 4: Fri. May 10, in-class20 questions, covers fission, fusion, particle physicsNo final exam.Essays returned FridayPhy107 Lecture 412Particles and fields• Think about electrons and photons• Modern view is that both are excitations of quantum‘fields’ that permeate all state.• Photon is an excitation of the photon field.• Electron is an excitation of the electron field.But there is a difference: Electron is a ‘matter particle’ Photon is a ‘force particle’Phy107 Lecture 413Quantum Electrodynamics: QED• A theory of electrons and photons• Electromagnetic force arises from exchange of photons.electronelectronphotonElectromagneticrepulsion via emissionof a photonPhy107 Lecture 414Question• An electron and anti-electron (positron)can annihilate, so that the particles‘vanish’. What is left after the annihilation?A. nothingB. quarksC. photonsD. neutrinosPhy107 Lecture 415Pair production, annihilation• Electron and positron can ‘annihilate’to form two photons.• Photon can ‘disappear’to form electron-positron pair.• Relativity: Mass and energy are the same– Go from electron mass to electromagnetic/photon energyPhy107 Lecture 416Electrons and photons• Electromagnetic force and electrons are both fields.• The fields have quanta: photon and electrons.• The Quantum field theory QED explains how theyinteract.• Very successful theory: explains perfectly all theinteractions between electrons and photons• Predicted a few things we didn’t expect:– Antiparticles - the positron.– Electrons and positions can be annihilated to photons andvice versa.2Phy107 Lecture 417Creating more particles• All that is needed to create particles is energy.• Energy can be provided by high-energy collision ofparticles. An example:– Electron and positron annihilate to form a photon.– This can then create particles with mc2<photon energy.e-e-,µ-e+e+,µ+First new particlefound this wayµ, Muon mass: 100MeV/c2,electron mass 0.5 MeV/c2Phy107 Lecture 418All the leptons• Six different leptons: cats and fleas• The cats have an electrical charge of -e• The fleas (neutrinos) have zero charge (neutral).Phy107 Lecture 419Three lepton generations• Each electron-like particle and neutrino paired ina generation.e—νeµ—νµτ—ντGeneration I Generation IIGeneration IIIPhy107 Lecture 4110QuestionThe difference between the differentgenerations of leptons isA. their chargeB. their massC. their colorD. their spinPhy107 Lecture 4111QuarksSix different kind of quarks,analogous to the six leptonsAll quarks have an electric charge,they couple to photon field.But they also have a ‘color’ charge,and they couple to the gluon field.Coupling of quarks to the gluonfield is the ‘strong’ interactionPhy107 Lecture 4112Three quark generations• Six different quarks, but two per generation– Just like the leptonsudGeneration IcsGeneration IItbGeneration IIICharge+2/3-1/33Phy107 Lecture 4113Quarks & leptons: matter particles• Three generationsdifferentiated primarily bymass (energy).• First generation– One pair of leptons,one pair of quarks• Leptons:– Electron, electron-neutrino.• Quarks:– Up, down.• All 3 generations seenPhy107 Lecture 4114Building combinations of quarks• Hadrons are composite particles made fromquarks• Baryons are hadrons with three quarks• Mesons are hadrons with two quarksQuestion: a proton is aA. MesonB. BaryonC. QuarkD. Fundamental particlePhy107 Lecture 4115Protons/Neutrons are compositePhy107 Lecture 4116QuestionA neutron is a composite particle consisting ofthree quarks (up=charge +2/3e, and down =charge -1/3e). Which is the structure of theneutron.A. up, up, upB. up, up, dowC. up, down, downTotal charge:+2e+e0Neutron has zero total chargePhy107 Lecture 4117Many baryonssudLambda (Λ)Q = 0M=1116 MeV/c2suuSigma (Σ+)Q = +1M=1189 MeV/c2sudSigma (Σ0)Q = 0M=1192 MeV/c2sddSigma (Σ−)Q = -1M=1197 MeV/c2u d sQMass+2/3 -1/3 -1/3Quark up down strangeu u d d ss~5 [MeV/c2] ~10 [MeV/c2] ~200 [MeV/c2]Excited state - Higher energy/mass Phy107 Lecture 4118Mesons• They are formed when a quark and an anti-quark“bind” together.• So far we’ve only seen 3 quark combinations. Thereare also 2 quark combinations.• The hadrons: 2 quarks, meson and 3 quarks, baryon.udWhat’s the charge of this particle?cdWhat’s the charge of this particle?Q=+1, and it’s called a π+Q= -1, and this charmmeson is called a D-sdWhat’s the charge of this particle?Q= 0, this strangemeson is called a K04Phy107 Lecture 4119How do particles interact?• The four fundamentalinteractions all haveassociated fields andmediating particles.• Talked about– EM interaction betweenelectrically charged particlesby exchange of photons– Strong interaction between‘color’ charged particles bygluon exchangePhy107 Lecture 4120EM interaction• Charged particles interact via theelectromagnetic (EM) interaction– A charged particle couples to the photon field– It excites a photon (excited state of photon field)and loses energy.– Another charged particle can absorb the energyfrom the photon field (photon disappears).Only particles with an electric charge couple to the photon field.Phy107 Lecture 4121Exchange particles• Each interaction has one or more associatedparticles that mediate the interaction.• The exchange particles are associated with theknown interactionsElectro- photon (1)magneticWeak W+, W- and Zo (3)Strong gluons (8)Gravity graviton (1)Interaction Mediating particle(s) Number• These all have integer spins ( bosons )Phy107 Lecture 4122The Standard Model• 6 leptons• 6 quarks• 4 interactions– Electromagnetic, Gravitational, Strong, Weak– ‘mediated’ by 13 exchange bosons, which areexcitations of the corresponding fields.Phy107 Lecture 4123QuestionWhich of the following particles is not a‘matter particle’A. ElectronB. QuarkC. GluonD. NeutrinoPhy107 Lecture 4124Many Charges• A particle with electrical charge couples to thephoton field• Particle with mass charge couples to grav. field• Particle with weak charge couples to weak int. field• Particle with ‘color charge’ couples to gluon field5Phy107 Lecture 4125Strong ForceIn the Standard Model particles are often
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