1Mon. Apr. 30, 2007 Phy107 Lect. 371From Last Time…• Particles are quanta of a quantum field– Represent excitations of the associated field– Particles can appear and disappear• Quarks and leptons– Three generations of leptons, quarks– Different only by masses• Today:– Making other particles from quarks– Quark interactionsEssay due WednesdayMonday office hours moved to Wed. this weekMon. Apr. 30, 2007 Phy107 Lect. 372Leptons: electron-like particlesCharge-e-e-eSpin1/21/21/2Mass (MeV/c2)0.51061,777ElectronMuonTauThese are referred to as three‘generations’ of particles.Difference between them is only massMon. Apr. 30, 2007 Phy107 Lect. 373Antiparticles• Each of these has an antiparticle,different only by charge.• Electron antiparticle= positron• Muon antiparticle= anti-muon• Tau antiparticle= anti-tauMatter and anti-matter canannihilate creating energyMon. Apr. 30, 2007 Phy107 Lect. 374Three neutrinosCharge000Spin1/21/21/2Mass (MeV/c2)~0~0~0Electron-neutrinoMuon-neutrinoTau-neutrinoThree ‘generations’ of particles.Both the muon and tau haveMuon-neutrinoTau-neutrino (detected in 2000)Mon. Apr. 30, 2007 Phy107 Lect. 375Six quarksCharge+(2/3)e-(1/3)eSpin1/21/2Mass (MeV/c2)36UpDown+(2/3)e-(1/3)e1/21/21,300100TopBottom+(2/3)e-(1/3)e1/21/2175,0004,300CharmedStrangeMon. Apr. 30, 2007 Phy107 Lect. 376The particle garden• Particle physics at this point has settled on acountable number of ‘fundamental particles’.• The bad news - there are:– (6 leptons +6 quarks)+(4 electroweak bosons +8 gluons +1 graviton) =25fundamental particles, not counting antiparticles!• The good news:– These are not just random, but have some relationshipsthat let us understand the ideas without thinkingimmediately about all the particles.2Mon. Apr. 30, 2007 Phy107 Lect. 377Three ‘generatations’ of particles• Three generationsdifferentiated primarilyby mass (energy).• First generation– One pair of leptons,one pair of quarks• Leptons:– Electron, electron-neutrino.• Quarks: Up, down.All have spin 1/2Mon. Apr. 30, 2007 Phy107 Lect. 378The generations of ‘matterparticles’LightHeavierHeaviestMon. Apr. 30, 2007 Phy107 Lect. 379Most of the mass is in the binding energy.Use up and down quarksu d sCharge QMass+2/3 -1/3 -1/3~5 [MeV/c2] ~10 [MeV/c2] ~200 [MeV/c2]Quark up down strangeu u d d ssuudProtonQ = +1M=938 MeV/c2dudNeutronQ = 0M=940 MeV/c2Mon. Apr. 30, 2007 Phy107 Lect. 3710 Making more composite particles• The forces which hold the protons and neutrons together inthe nucleus are VERY strong.•They interact via the STRONG FORCE.• Protons and neutrons are among a class of particles called“hadrons” (Greek for strong).• Particles made quarks.• Baryons are hadrons which contain3 quarks (no anti-quarks).• Anti-baryons are hadrons which contain3 anti-quarks (no quarks).Mon. Apr. 30, 2007 Phy107 Lect. 3711Are there baryons other thanprotons and neutrons?Other quarks can combine to form other baryons. For example:usdThis combination is called a Lambda baryon, or Λ0 for shortCharge 0:(2/3) + (-1/3) + (-1/3) = 0Mon. Apr. 30, 2007 Phy107 Lect. 3712More 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/mass3Mon. Apr. 30, 2007 Phy107 Lect. 3713Another baryon:uDelta baryon, or Δ++ for short.Charge: +2(2/3) + (2/3) + (2/3) = +2 !uuWhat’s this baryon’s electriccharge?A. 0B. +1C. +2Mon. Apr. 30, 2007 Phy107 Lect. 3714Mesons• 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 K0Mon. Apr. 30, 2007 Phy107 Lect. 3715Leptons and quarks:what’s the difference?• One important difference between leptons andquarks is how they interact.Which of these interactions are NOT common toboth leptons (e.g. electrons) and quarks.A. Electromagnetic forceB. gravitational forceC. strong forceMon. Apr. 30, 2007 Phy107 Lect. 3716• We said the Coulomb interaction is betweenparticles with electrical charge.• Understood by exchanging photons.• The other interactions:– Weak– Strong– GravitationalMon. Apr. 30, 2007 Phy107 Lect. 3717Interactions between particles• The modern view of forcesis in terms of particle exchange.• These are ‘virtual’ particles of the fieldscreated by the particle charges.This shows Coulombrepulsion between twoelectrons. It isdescribed as theexchange of a photon.Mon. Apr. 30, 2007 Phy107 Lect. 3718Four fundamental forces4Mon. Apr. 30, 2007 Phy107 Lect. 3719Exchange bosons• 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 )Mon. Apr. 30, 2007 Phy107 Lect. 3720Electromagnetic ForceParticles are often classified using the forces bywhich they interact.Which of these particle does not interact with theelectromagnetic force?A. electronB. muonC. photonD. quarkMon. Apr. 30, 2007 Phy107 Lect. 3721Charge• How do we know which interactions apply?• The particle must be able to couple to the fieldof the “mediating” particle• Example:– When a photon is exchanged between two particles,there is a electromagnetic or Coulomb force.– We know that only particles with electrical chargeinteract via the Coulomb force– Zero charge -> zero Coulomb interactionMon. Apr. 30, 2007 Phy107 Lect. 3722Many Charges• In this language, we say that theelectrical charge is a ‘source’ of an EM field.• A mass ‘charge’is the source of a gravitational field• A weak ‘charge’ (sometimes called ‘flavor’)is the source of a weak interaction field• A strong ‘charge’ (sometimes called ‘color’)is the source of a strong interaction fieldMon. Apr. 30, 2007 Phy107 Lect. 3723‘Electric Charge’What does it really mean for a particle to have electric charge ?It means the particle
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