Most of Modern Physics today isconcerned with the extremes of matter:• Very low temperatures, very largenumbers of particles, complex systemsÆ Condensed Matter Physics• Very high temperatures, very largedistancesÆ Astrophysics, Cosmology• Very small distances, very highenergiesÆ Elementary Particle Physics (High Energy Physics)The fundamental particles(so far)• Electron: charge -1, doesn’t feel strongforce• Proton: charge +1, feels strong force• Neutron: charge 0, feels strong force• Positron: (the anti-electron)• Same mass and opposite charge asthe electron.• Predicted in 1928 by Dirac based onrelativistic generalization of theSchrodinger equation.• Discovered in Cosmic Rays in 1932.(All particles have antiparticles. Theanti-proton was discovered in 1956)Cosmic Rays• Cosmic rays are very energeticparticles, mostly protons, that comefrom interstellar space.• They collide with particles in theearth’s atmosphere, producing showersof very high energy particles.• Their energies can be as high as1021 eV, about a billion times thehighest energy human-builtaccelerator.The fundamental particles(so far) (continued)• Neutrino:• charge 0, doesn’t feel strong force.• Predicted by Pauli in 1930, in orderto conserve momentum in nuclear bdecay.• Discovered in 1956.• Photon: charge 0, associated with theelectromagnetic forceThe photon carries or mediates the EMforce by being exchanged “virtually”between charged particles. This isrepresented in Feynman diagrams:e-e-e-e-gtThe prediction of the Pion1935- Hideki Yukawa :Based on analogy with the photon asmediator of the EM force, Yukawaargued that there also should be aparticle that mediates the strongforce.The Mass of Yukawa’s particle (the Pimeson or pion) can be estimated by theuncertainty principle:Range of nuclear force is Dx ~ 2 fm.A virtual pion travels this distancein roughly time Dt ~ Dx/c.The uncertainty in Energy necessary forthe pion to exist for this amount of timeis: DE ~ mpc2 ~ h/Dt = hc/(2 fm) ~ 100 MeV(note: mp > mp > me )Only 2 years after Yukawa’s prediction, anew particle was discovered in cosmic rayswith just the right mass. But it was notYukawa’s particle!!! (more on this later.)ppnnpt1947 - Yukawa’s pion finally discovered inin cosmic rays.It comes in three varieties:• Charged pions p±, with charge ±1 andmass 140 MeV/c2. They are anti-particles of each other. They live witha mean lifetime of 2.6x10-8 secondsbefore decaying to lighter particles.• The neutral pion p0, with charge 0 andmass 135 MeV/c2. It is its own anti-particle. It lives about 8.4x10-17seconds before decaying into twophotons.ppnnp0nppnp±More Particles1938 - Muon discovered.• Its mass was 106 MeV/c2.(just right for Yukawa’s particle )• But subsequent experiments showedthat it did not interact strongly,passing easily through dense matter.(not right for Yukawa’s particle)In many ways the muon (charge ±1)behaves like a heavy electron.“Who ordered that?”- I.I. RabiMany other new particles found in cosmicrays:K-meson (Kaon) and the L-Baryon (heavierthan the proton). These had some“Strange” properties, such asunexpectedly long life-times.In 1950’s more discoveries:S-Baryons and h-mesons, and many more!The particle zoo is getting crowded!Some organization is needed.ForcesGravity: Important in everyday lives andin astronomical phenomena, butnegligible for elementary particles.Electromagnetic: Electricity andMagnetism unified into a singlefundamental interaction by Maxwell.The force carrier is the photon, whichcan extend over long range.Strong: Holds protons and neutronsinside nuclei. Very strong, but shortrange. Pion can be considered to carrythe force, but a more fundamentaldescription will come later.Weak: A very short range force, which isresponsible for b-decay of nuclei, andthe decay of many other elementaryparticles.Classification of ParticlesThere are three broad categores:Leptons: Particles such as electrons,muons, and neutrinos, which do not feelthe strong force. Leptons always havespin 1/2 h.Hadrons: Particles which do participate instrong interactions. (any spin)Gauge particles (Gauge Bosons): Theparticles responsible for carrying theforces. The only one we have met sofar is the photon.LeptonsThere are believed to be six leptons(along with their associated anti-leptons).They come in three generations (pairingsof a charged lepton and a neutrino).Generation Particle Charge Mass e -1 0.5 MeV/c2 1 ne 0 ~ 0 m -1 106 MeV/c2 2 nm 0 ~ 0 t -1 1784 MeV/c2 3 nt 0 ~ 0• The t (Tau) lepton was discovered byMartin Perl and collaborators at theStanford Linear Accelerator (SLAC) in1976.• Heavier charged leptons decay to thelighter ones. For example:m- Æ e- + nm + ne(The t can also have hadrons in its decay.)• In the last couple years it has beenverified that neutrinos do have a mass(although very small). This was seenindirectly through oscillations from onetype of neutrino to another. Theseoscillations can only occur if theneutrinos have nonzero mass.HadronsHadrons feel the strong force. They canbe further subdivided into Baryons andMesons.Mesons are hadrons with integral spin(mostly 0 or 1, but sometimes 2 orhigher). Most have masses betweenthat of the electron and proton. Thepion (p), Kaon (K), and eta meson (h)are examples.Baryons are hadrons with 1/2 integral spin(mostly 1/2, but sometimes 3/2 orhigher). The lightest baryons are thenucleons (proton and neutron).Force ParticlesEach of the four basic forces is mediatedby the exchange of a force particle.Force ParticleElectromagnetic photon (g)Strong nuclear pion (p) *Weak nuclear IntermediateBoson (W±, Z)Gravity Graviton*The modern, more fundamentalformulation of the strong force has thegluon (g) as the carrier, as we shall see.1934 - W particles were first proposed byFermi.1982 - W± and Z particles discovered atCERN.Gravitons not yet observed directly.pne-neW-tConservation LawsCertain quantities are always conserved.In addition to energy, momentum, andelectric charge, they are:Baryon number: The generalization ofconservation of nucleons (each withbaryon number 1). Anti-Baryons havebaryon number -1. Mesons, leptons andgauge particles have baryon number 0. e- + p Æ e- + p + n + nBaryon # 0 + 1 = 0 + 1 + 1 + (-1)Lepton number: