1Fri. Apr. 28, 2006 Phy107 Lecture 381From Last Time…• Discussed the weak interaction• All quarks and leptons have a ‘weak charge’– They interact through the weak interaction• Weak interaction often swamped byelectromagnetic or strong interaction.• Most clearly manifested in particle decays,where the weak interaction can change oneparticle into another.Course evaluations on MondayFri. Apr. 28, 2006 Phy107 Lecture 382QuestionWhich of the following particles is not a‘matter particle’A. ElectronB. QuarkC. GluonD. NeutrinoFri. Apr. 28, 2006 Phy107 Lecture 383Matter particles & force carriers• Matter Particles– Quarks and leptons– These are fermions(half-integer spin)• Force pariticles– These are all bosons– (integer spin)Fri. Apr. 28, 2006 Phy107 Lecture 384QuestionWhich of the following particles cannot interactwith a paticle that carries only the weakcharge?A. QuarkB. NeutrinoC. ElectronD. All of them canFri. Apr. 28, 2006 Phy107 Lecture 385Fundamental Matter ParticlesThree generations ofleptons and quarksAll feel theweak force.Charged leptons feelweak & EM forcesQuarks feelweak, EM, & strongFri. Apr. 28, 2006 Phy107 Lecture 386Quarks and the weak force• Quarks have color charge, electric charge, and weak charge— other interactions swamp the weak interaction• But similar to leptons, quarks can change their flavor(decay) via the weak force, by emitting a W particle.udGeneration IcsGeneration IIEmit W+2x10-12 secondstbGeneration IIIEmit W+10-23 secondsCharge+2/3-1/32Fri. Apr. 28, 2006 Phy107 Lecture 387Flavor change between generations• But for quarks, not limited to within a generation• Similar to leptons, quarks can change their flavor (decay)via the weak force, by emitting a W particle.udGeneration IcsGeneration IIEmit W-tbGeneration IIIEmit W-10-12 secondsCharge+2/3-1/3Fri. Apr. 28, 2006 Phy107 Lecture 388‘Combining’ the quarks• In this sense, different quarks can beconsidered as different ‘manifestations’ ofthe same particle.• Just as we wouldn’t call spin-up and spin-down electrons different particles, thedifferent flavor quarks don’t have to beconsidered as different particles.• Three generations, six quarks, can beconsidered different ‘facets’ of same field.• Why six? This is only an input to the theory.Fri. Apr. 28, 2006 Phy107 Lecture 389Actual quark decays• Quarks are bound into hadrons– bayrons (3 bound quarks), mesons(2 bound quarks).• But the internal quarks can still emit W particlesand change into something else.• Most of the time, hadrons containing heavyquarks (c, s, b, t) decay by ‘emitting’ a W andtransforming into the next lightest quarkFri. Apr. 28, 2006 Phy107 Lecture 3810Decay of heavy quarksTop quark decays so fast (10-23 s), it doesn’t have time to form a meson. t b + t+ + nttW +τ +ντbB— particle decayswithin 1.5x10-12 s. B- D0 + µ- + νµbW -µ -νµcuuThe D0 meson decayswithin 0.5x10-12 s.This decay: D0 K- + e - + νecW +e +νesuuFri. Apr. 28, 2006 Phy107 Lecture 3811Can create anything!• Showed W decaying into leptons.• But W can create any particles that– Have a weak charge (couple to the weak force)– Conserve charge– Conserve energy (mass)• W commonly decays into two quarks:W+ud(+2/3)(+1/3)M~91 GeV/c24 GeV/c28 GeV/c212 GeV/c2W-du(-2/3)(-1/3)M~91 GeV/c28 GeV/c24 GeV/c212 GeV/c2Fri. Apr. 28, 2006 Phy107 Lecture 3812What happens to the quarks?• The quarks form a bound state.• In simplest case, the two quarks from Wdecay bind to form a meson– For instance:W+udThis is a π+ meson3Fri. Apr. 28, 2006 Phy107 Lecture 3813But it can be more complicatedπ0π-K-K+Hadrons!Think of the gluons being exchanged as aspring… which if stretched too far, will snap!Use stored energy in spring to create mass. duuduuduuuuudsusudduusddduuW-Stretch the spring:turn kinetic intopotential energyMore stretch,more storedenergy.Spring ‘snaps’.Use energy tocreate uu pairFri. Apr. 28, 2006 Phy107 Lecture 3814Basic ideas• As far as the weak interaction goes, leptons andquarks are basically identical.• All carry a weak charge.• W particles can decay into anything consistentwith charge and energy conservation• So maybe all quarks and leptons are justdifferent ‘states’ of the same ‘master particle’Fri. Apr. 28, 2006 Phy107 Lecture 3815Similar particles…• All six quarks can change flavor via the weakinteraction. Within a generation or betweengenerations.• Leptons can change flavor within a generation,and neutrinos between generations(discoveredrecently by looking at neutrinos from the sun).• View 12 fundamental matter particles as onlytwo different types:different flavors are just diff. ‘orientations’.Fri. Apr. 28, 2006 Phy107 Lecture 3816And several different interactions• Remember that interactions are due toexchange of bosons.• EM interaction - exchange photons• Weak interaction - exchange W+, W—, Zo• Strong interaction - exchange gluons (8)Fri. Apr. 28, 2006 Phy107 Lecture 3817Unification• Can unify the particles to some extent (notreally accomplished yet)• But haven’t yet about unifying the fourforces (13 force-carrying bosons)• ‘Standard model’ has successfully unifiedtwo the four forces• Electromagnetic and Weak force unified into‘electroweak’.Fri. Apr. 28, 2006 Phy107 Lecture 3818The next steps• Comparison of forces• Unification of the electromagnetic and theweak force– the Standard Model• Where do particles get mass?– The Higgs mechanism and the Higgs boson(s)• Prospects for unifying all of particle physics– Grand Unified Theories4Fri. Apr. 28, 2006 Phy107 Lecture 3819Exchange Bosons (force carriers)EM WeakFri. Apr. 28, 2006 Phy107 Lecture 3820Electro-weak unificationThe standard model says thatthe electromagnetic interaction (photon exchange) &the weak interaction (W+, W-, Zo exchange)are different pieces of the same electroweak interactionNeutral weak Electromagnetic• Zero charge• Mass=91 GeV/c2• Range ~ 10-18 m• Zero charge• Mass=0 GeV/c2• Range ~ inf.Pos. weak Neg. weak• Pos. charge• Mass=80 GeV/c2• Range ~ 10-18 m• Neg. charge• Mass=80 GeV/c2• Range ~ 10-18 mW+W-νeνeνeνeFri. Apr. 28, 2006 Phy107 Lecture 3821Some similarities hereThese two bothexchange neutral bosonsNeither bosonchanges the lepton flavor(remains electron)Neutral weak Electromagnetic• Zero charge• Mass=91 GeV/c2• Range ~ 10-18 m• Zero charge• Mass=0 GeV/c2• Range ~ inf.These two
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