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UW-Madison PHYSICS 107 - Phy 107 Lecute 35 Notes

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1Phy107 Fall 20061From Last Time…• Particles are quanta of a quantum field– Represent excitations of the associated field– Particles can appear and disappear• Particles interact by exchanging other particles– Electrons interact by exchanging photons• This is the Coulomb interaction• Electrons are excitations of the electron field• Photons are excitations of the photon field• TodayMore particles!Essay due FridayPhy107 Fall 20062Something unexpected• Raise the momentum and the electrons and seewhat we can make.• Might expect that we make a quark and anantiquark. The particles that make of the proton.– Guess that they are 1/3 the mass of the proton 333MeVe-µ-e+µ +Instead we get amuon, acts like aheavy version ofthe electronµ, Muon mass: 100MeV/c2,electron mass 0.5 MeV/c2Phy107 Fall 20063Accelerators• What else can we make with more energy?• Electrostatic accelerator: Potential difference Vaccelerate electrons to 1 MeV• Linear Accelerator: Cavities that make EM wavesparticle surf the waves - SLAC 50 GeV electrons• Cyclic Accelerator: Circular design allows particlesto be accelerated by cavities again and again– LEP 115 GeV electrons– Tevatron 1 TeV protons– LHC 7 TeV protons(starts next year)Phy107 Fall 20064CERN (Switzerland)27 km•CERN, GenevaSwitzerland• LHC Cyclicaccelerator• 27km, 14TeV7+7=14Phy107 Fall 20065Detectors are required todetermine the results ofthe collisions.• CDF: ColliderDetector Facilityat FermilabMeasuring particle collisionsPhy107 Fall 20066Fundamental ParticlesIn the Standard Model the basic building blocks aresaid to be ‘fundamental’ or not more up ofconstituent parts.Which particle isn’t ‘fundamental’:A. electronB. muonC. photonD. proton2Phy107 Fall 20067What have we learned?Matter is made of atomsLeptonseeQuarksdu“ Atoms are made of leptons and quarks “Atoms are made of leptons and quarksInteract via different forces carried by particles,photons…Phy107 Fall 20068Neutrino• Neutrino was first detected in 1956• The neutrino has–Zero charge,spin 1/2,almost zero mass• Electron has– Charge -e,spin 1/2,mass 0.5 Mev/c2• Electron and neutrinogenerically called leptonsSeen before in the weak interaction.Fred Reines, Clyde Cowan, at theHanford nuclear reactorPhy107 Fall 20069Protons/Neutrons are compositePhy107 Fall 200610Protons, neutrons, and quarks• Protons and neutrons are made up of– ‘up’ quarks and ‘down’ quarksCharge+(2/3)e-(1/3)eSpin1/21/2Mass (MeV/c2)36UpDownPhy107 Fall 200611QuestionA 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, downC. up, down, downTotal charge:+2e+e0Neutron has zero total chargePhy107 Fall 200612Protons & NeutronsTo make a proton:We bind 2 up quarks of Q = +2/3and 1 down quark of Q = -1/3. The total charge is 2/3 + 2/3 + (-1/3) = +1 !To make a neutron:We bind 2 down quarks of Q= -1/3with 1 up quark of Q = +2/3 to get: (-1/3) + (-1/3) + (2/3) = 0 !3Phy107 Fall 200613Hierarchy of structureR ~ 10-15 m (strong)protons and neutrons aremade from quarksR ~ 10-10 m (electromagnetic)Atoms are made from protons,neutrons, and electronsR > 106 m (gravitational)We’ll talk about the rest ofthe universe laterLeptonseeQuarksduPhy107 Fall 200614What about the muon?• The muon found early on.– Heavy version of the electron.• Otherwise would have been fairly simple!e-µ-e+µ +µ, Muon mass: 100MeV/c2,electron mass 0.5 MeV/c2Phy107 Fall 200615More particles• In 1975, the tau lepton was discovered atSLAC.(Nobel prize, 1995)Just like the electron and muon, but 3500 timesheavier than the electron– Same charge,same spin,but different mass.• Made this particle bycreating more energy.Both the muon and tau haveMuon-neutrinoTau-neutrino(detected in 2000)Stanford linearaccelerator center (SLAC)Phy107 Fall 200616The 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.Phy107 Fall 200617Three 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 massPhy107 Fall 200618Three neutrinosCharge000Spin1/21/21/2Mass (MeV/c2)~0~0~0Electron-neutrinoMuon-neutrinoTau-neutrinoThese are referred to as three‘generations’ of particles.Difference between them is only mass4Phy107 Fall 200619Six 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)e1/21/2175,0004,300CharmedStrangePhy107 Fall 200620Quarks: Heavy, Heavier, Heaviest 6 different kinds of quarks. Matter is composed mainlyof up quarks and down quarksbound in the nuclei of atoms. Masses vary dramatically(from ~0.005 to 175 [GeV/c2]) Heavier quarks are unstable,rapidly decay to lighter quarksupdownstrangecharmbottomtop0.0010.010.1110100100001234567Mass [GeV/c2]Gold atomSilver atomProtonExample: t b (~10-23 [s]) b c (~10-12 [s]) c s (~10-12 [s]) s u (~10-7-10-10 [s])More on quark decays later…Phy107 Fall 200621Three ‘generations’ 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 1/2Phy107 Fall 200622The generations of ‘matterparticles’LightHeavierHeaviestPhy107 Fall 200623Antiparticles• 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 energyPhy107 Fall 200624 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 of quarks.• Baryons are hadrons which contain3 quarks (no anti-quarks).• Anti-baryons are hadrons which contain3 anti-quarks (no quarks).5Phy107


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UW-Madison PHYSICS 107 - Phy 107 Lecute 35 Notes

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