1Phy107 Fall 20061• The hierarchy or matter– Large to small: tables, molecules, atoms, proton neutrons electrons– Previously discussed protons neutrons and electrons and the forcesto understand matter• Would like to understand what is fundamental. Thesmallest indivisible building blocks and the forces to putthem together• Quarks and leptons and the four forces– Whole structure of matter can be understood from the quarks andleptons - along with the forces– Understood in terms of quantum field theory of particles, fields andforcesFrom the last timeEssay due Friday: Please include citation to referenceHW 10: Chap 18 - Conceptual 6, 10, 12 - Problems 2, 4Phy107 Fall 20062What have we learned?Matter is made of atomsLeptonseeQuarksdu“ Atoms are made of leptons and quarks “Atoms are made of leptons and quarksInteract via different forces carried by particles,Electromagnetic - photon, also strong, weak, gravityPhy107 Fall 20063Generations - Times 3• Three generationse—eGeneration Iduµ—µ—Generation IIGeneration IIIscbtCharge+2/3-1/3-10Phy107 Fall 20064QuestionThe difference between the differentgenerations of leptons or quarks isA. their chargeB. their massC. their colorD. their spinPhy107 Fall 20065The Standard Model• 6 leptons•6 quarks•Today: 4 interactions– Electromagnetic, Gravitational, Strong, Weak– ‘mediated’ by 13 exchange bosons, which areexcitations of the corresponding fields.Phy107 Fall 20066EM 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.2Phy107 Fall 20067Strong force and the QuarksSix 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 Fall 20068Quarks & Gluons1. There are three color charges2. Gluons are the carrier of the strong force3. They keeps quarks bound up inside hadrons4. Gluons themselves carry color, they can interact with each other!uuuPhy107 Fall 20069Quark interactions: gluons• Gluon carries colorcharge.• So when a quark emits agluon, it changes color.• But this also means thatgluons can interact viathe color force.Each of the 8 color combinationshave a “color” and an “anti-color”Phy107 Fall 200610Interactions through Exchangeof Color ChargergrgrgInitially After gluon emission RED RED-ANTIGREEN + GREEN(quark) (gluon) (quark)Emission of Gluon Before gluon absorption After gluon absorptionRED-ANTIGREEN + GREEN RED (gluon) (quark) (quark)Re-absorption of GluonPhy107 Fall 200611Why Three Colors?uThe quarks are fermions:Fermions are identical and are not allowed tobe in the same state. The wave functionwould disappear.The electrons in an atom could be in the sameenergy state if they had different spins.The same same is true of three quarks ifsomething is different about them. Color!uuRemember the Delta baryon, or ++Phy107 Fall 200612Feynman Diagrams (Quark Scattering)Quark-antiquarkAnnihilationgQuark-quarkScatteringCould also beQuark-antiquarkScatteringorAntiquark-antiquarkScatteringtimePositiondduuguuddud3Phy107 Fall 200613Gluon interactionsSince gluons carry “color charge”, they can interact with each other !(Photons can’t do that)gggggGluon-gluon ScatteringggggGluon-gluon FusiongPhy107 Fall 200614Gluons – Important Points Gluons are the “force carrier” of the strong force. They only interact with object which have color, or color charge. Therefore, gluons cannot interact with leptonsbecause leptons do not have color charge !qe+ge -qThis cannot happen, becausethe gluon does not interact withobjects unless they have color charge! Leptons do not have color charge !Phy107 Fall 200615Gluons in the hadrons.The gluons are all overinside hadrons!! In fact there are a lot morethan shown here !!! That’s where the extra masscomes from. u and d quarks are0.003 and 0.006 and the proton1 GeVuud~10-15 [m]ProtonPhy107 Fall 200616ConfinementPhy107 Fall 200617ComparisonStrong and EM forcered, green, blue+, -Charge typesAll objects withcolor chargeAll objects withelectrical chargeMediates interactionbetween:10-14 [m](inside hadrons)Infinite ( 1/d2)RangeYes, color chargeNoneCharge ?00MassGluon (g)Photon ()Force CarrierStrongEMPropertyPhy107 Fall 200618The weak interaction• Weak interaction is ‘not strong’, but is important. It can change one particle into another!• Muon, tau, can turn into leptons and neutrinos!• Quarks can turn into other quarks• Particles with a ‘weak charge’(quarks and leptons)couple to the ‘weak field’• Excitations of the weak fieldare the Z and W bosons.4Phy107 Fall 200619Importance of weak interaction• Weak interaction is much weakerthan strong and electromagnetic interactions• But it’s existence is of great importance, as well as itsstrength.• Fusion in the sun:– First part is hydrogen ->into heavy hydrogen (deuterium),– Caused by by the weak force. Without this force solar energyproduction would not be possible.– If weak force been stronger, sun’s life span too short for life tohave had time to evolve on any planet!• Practical applications– Radioactive elements used in medicine and technology (Usuallybeta-radioactive), and in the beta-decay of carbon-14 (carbon-dating)Phy107 Fall 200620QuestionQuarks interact only via theA. weak forceB. strong forceC. gravitational forceD. all of the abovePhy107 Fall 200621Carriers of the weak force• Like the Electromagnetic & Strong forces, the Weak force is also mediated by “force carriers”.• For the weak force, there are three force carriers:W+W-Z0These “weak force” carrierscarry electric charge also !This “weak force” carrieris electrically neutralThe “charge” of the weak interaction is called “weak charge”Phy107 Fall 200622Massive particles• The W+, W-, and Zo are very massiveW+W-Zo80.4 Mev/c291.2 Mev/c2W and Z have almost half the mass of the topquark, the heaviest fundamental particlePhy107
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