1Wed. Apr. 26, 2006 Phy107 Lect. 361From Last Time…• Discussed quarks and their interactions• Six quarks, paired in three generations• Quarks have electric charge, participate in EMinteraction by exchanging photons.• Quarks also have color charge, participate in colorinteraction by exchanging gluons.• Color charge as six varieties– Red, green, blue, anti-red, anti-green, anti-blue– Like colors repel, unlike colors attract.Wed. Apr. 26, 2006 Phy107 Lect. 362The Standard Model• 6 leptons• 6 quarks• 4 interactions– Electromagnetic, Gravitational, Strong, Weak– ‘mediated’ by 13 exchange bosons, which areexcitations of the corresponding fields.Wed. Apr. 26, 2006 Phy107 Lect. 363Leptons• The cats have an electrical charge of -e• The fleas (neutrinos) have zero charge (neutral).Wed. Apr. 26, 2006 Phy107 Lect. 364Lepton generations• Each electron-like particle and neutrino paired ina generation.e—νeµ—νµτ—ντGeneration I Generation IIGeneration IIIWed. Apr. 26, 2006 Phy107 Lect. 365QuestionThe difference between the differentgenerations of leptons isA. their chargeB. their massC. their colorD. their spinWed. Apr. 26, 2006 Phy107 Lect. 366EM 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.2Wed. Apr. 26, 2006 Phy107 Lect. 367QuarksSix 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.This is the ‘strong’ interactionWed. Apr. 26, 2006 Phy107 Lect. 368Quark generations• Six different quarks, but two per generation– Just like the leptonsudGeneration IcsGeneration IItbGeneration IIICharge+2/3-1/3Wed. Apr. 26, 2006 Phy107 Lect. 369Four interactions• 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 exchangeWed. Apr. 26, 2006 Phy107 Lect. 3610The weak interaction• Weak interaction is ‘not strong’, but is important. It can change one particle into another!• Muon, tau, can turn into neutrinos!• Quarks can turn into other quarks• Particles with a ‘weak charge’couple to the ‘weak field’• Excitations of the weak fieldare the Z and W bosons.Wed. Apr. 26, 2006 Phy107 Lect. 3611• "Although the weak interaction is much weaker thanboth the strong and the electromagneticinteractions, it is of great importance in manyconnections. The actual strength of the weakinteraction is also of significance. The energy of thesun, all-important for life on earth, is producedwhen hydrogen fuses or burns into helium in a chainof nuclear reactions occurring in the interior of thesun. The first reaction in this chain, thetransformation of hydrogen into heavy hydrogen(deuterium), is caused by the weak force. Withoutthis force solar energy production would not bepossible. Again, had the weak force been muchstronger, the life span of the sun would have beentoo short for life to have had time to evolve on anyplanet. The weak interaction finds practicalapplication in the radioactive elements used inmedicine and technology, which are in general beta-radioactive, and in the beta-decay of a carbonisotope into nitrogen, which is the basis for thecarbon-14 method for dating of organicarchaeological remains. ... Of special interest is aresult, published in the summer of 1978, of anexperiment at the electron accelerator at SLAC inStanford, USA. In this experiment the scattering ofhigh energy electrons on deuterium nuclei wasstudied and an effect due to a direct interplaybetween the electromagnetic and weak parts of theunified interaction could be observed."Wed. Apr. 26, 2006 Phy107 Lect. 3612QuestionQuarks interact only via theA. weak forceB. strong forceC. gravitational forceD. all of the above3Wed. Apr. 26, 2006 Phy107 Lect. 3613The Weak ForceEMSTRONGWEAKWed. Apr. 26, 2006 Phy107 Lect. 3614Carriers 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”Wed. Apr. 26, 2006 Phy107 Lect. 3615Massive 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 particleWed. Apr. 26, 2006 Phy107 Lect. 3616What interacts?• Any particle with a ‘weak charge’will interact via the weak interaction.• All quarks and leptons carry a weak charge.• The weak interaction occurs by exchanging aW+ , W-, or Zo• But all quarks have electric charge,and half the leptons do.• In this case, weak interaction overwhelmed byelectromagnetic interaction.Wed. Apr. 26, 2006 Phy107 Lect. 3617The NeutrinoWhy can the neutrino pass through light years ofmaterial without interacting with anything.A. It has no massB. It is very smallC. It interacts only via the weak forceD. It interacts only via the EM forceWed. Apr. 26, 2006 Phy107 Lect. 3618Neutrinos• Neutrino has no electric charge• Interacts only via the weak force.• How weak is weak?– Neutrino traveling in solid lead would interactonly once every 22 light-years!– And weak force only “kicks in” for d <10-18 m, adistance ~ 1000 times smaller than the nucleus• But there are lots of neutrinos, so it ispossible to observe an interaction.4Wed. Apr. 26, 2006 Phy107 Lect. 3619Detecting neutrinos• Neutrinos interact with all matter,since all matter particles have a weak charge.• But the interaction is extremely weak• Need large volumes, sensitive detectors, to see neutrinos.Examples of neutrino detectors:Super Kamiokande (Japan)Ice Cube (UW-Madison at Antartica)Wed. Apr. 26, 2006 Phy107 Lect. 3620Ice CubeWed. Apr. 26, 2006 Phy107 Lect. 3621Wed. Apr. 26, 2006 Phy107 Lect. 3622AMANDA / Ice Cube• 6 minute movieWed. Apr. 26, 2006 Phy107 Lect. 3623Exchanging the W• W+ (pos. electric
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