L19: Particle PhysicsFundamenal Particles - Version 1.0 All matter composed of atoms Nucleus composed of ‘nucleons’, I.e. protons and neutrons• Feel strong nuclear force which keeps them in the nucleus• Both have very similar mass Nucleus surrounded by electron ‘shells’ or ‘clouds’ which reflect theenergy levels of the electrons in that atom Beta decay of some atoms Leads to idea of a ‘neutrino’YYYYweak------~00NeutrinoYNmproton/200-1electronNY~mproton0neutronYYmproton+1ProtonelectromagneticStrongMasschargeAntimatter Dirac, 1930 Produced a relativistic model of quantum mechanics Includes special, not general, relativity Noticed a ‘symmetry’ about equations Negatively charged electron moving forward in time same aspositively charged electron (‘positron’) moving backwards in time So nature requires existence of ‘anti-electron’, what we call a positron Positrons Observed in cosmic ray experiments• A particle created by another particle from space and whichinteracts with a detector• Produces a particle with the same mass as an electron, butopposite charge Whenever near ordinary matter• It interacts with atoms by annihilating an electron and producingphotons Hypothesize anti-proton and anti-neutron: they have been observedMesons Meson theory: Yukawa, 1931 Think of strong force as exchange of mediating particle (‘meson’) Reason strong force is restricted in distance it can reach• Meson has mass: by uncertainty principle (ΔEΔt>h/2), avirtual meson can only exist for a length of time, Δt• By observing range of strong force, can estimate mass of‘meson’ to be ~mproton/7 ‘mu meson’ or ‘muon’ discovered: birth of particle physics Mass ~ mproton/9 Looks like a heavy electron -- it doesn’t feel the strong force!• So can’t answer the problem of the strong force’s range Pi mesons discovered 1947 and 1950 Feel strong force, mass ~ mproton/7 Many mesons discovered since 1950Decays Almost all particles decay to two or more daughter particles Neutrons, pions, muons, all other mesons Exceptions: proton, electron, neutrino Reaction rules Conserve charge, total energy, momentum Also observe that ‘lepton number’, L is conserved• E.g. if one muon (L = +1) decays, then the sum of L for the daughters muststill be +1 Also observe heavier versions of proton and neutron• Called ‘baryons’ since they are heavy• Observe that reactions conserve ‘baryon number’, B• Eg. if one neutron decays (B=+1), then the daughters must sum to B=+1 Note: mesons have L = 0, B = 0, antiparticles have opposite sign L and B10-16 sπ0 → γ + γ (strong)10-8 sπ+ → µ+ + νµ(weak) 10-6 sµ+ → e+ + νe + νµ (weak) Lifetime (half-life)DecayMuch faster because it’sa strong interactionInteractions as Momentum Exchange conservation of momentum think of forces as interactions– two particles interact by exchanging a messenger particle• eg. electronmagnetism uses the photon• exchanged particle• transfers momentum from one interacting particle to anotherThink of two skaters - one throws heavy ball to another - thrower looses momentum - receiver gains momentumspacetimeparticle 2particle 1messengerFour Fundamental Interactions Four forces generalized to ‘interactions’ gravity MUCH weaker than others, but it’s long-range, there’s lots of matter, and most matter charge neutralgravity mass ∞ 10-38weak flavor 10-1510-5electro magne t i ccharge ∞ 0.01force coupling range(cm) strengthstrong color 10-181Strange Particles and Quarks In 1950s, observed a new class of particles K meson, ‘kaon’ was first, but new baryons also found always made in pairs Some might have a proton or neutron as end product of decay• So they have a different ‘flavor’ than a proton or neutron• Called ‘strange’ particles Quark model A pattern in the charge, mass and flavor properties of mesons andbaryons was observed• Suggested these particles were composed of smaller, morefundamental pieces: ‘quarks’ Further evidence Collide electrons onto protons Like Rutherford experiment See large deflections of electrons Therefore, proton has substructureΣ+-1+1UusNeutron, Δ000Uddproton, Δ+0+1UudparticlesStrangenessChargequarksStrong Interactions strength of strong interaction increases with increasing distance• opposite to gravity and electromagnetism when have two colored particles interacting nature does not permit ‘naked’ color energy in strong interaction• grows as particles move apart when energy greater than masses of fundamental particles• a ‘jet’ of particles ‘pulled’ out of the virtual sea or vacuum• extremely messy and poorly understood processWeak Interactions gluons (strong interaction) and photons (electromagneticinteraction) are massless By uncertainty principle (ΔEΔt > h/2), they can exist out ofthe quantum sea for infinite amount of time Weak force carried by W and Z particles Very massive Can only exist briefly, so travel only very short distances This is why weak force is ‘weak’, and why it is restricted to short distancesElectroweak Unification electromagnetic and weak interactions different manifestations of same, more fundamental force why so different in strength?• due to mass of exchange particle (W and Z)• restricts distance over which weak interactions can occur• only different in strength at low energies• at higher energies, W and Z massless so how do the W and Z acquire mass? the Higgs analogy: iron atoms will align themselves at low temperatures• despite no direction preferred in interaction between atoms• therefore atoms acquire a certain energy• ie. must add heat to break the alignment lowest energy state of universe• nonzero Higgs field• generates mass for W and ZNew Particles Problems with electroweak model If only three quarks (up, down, strange)• get infinities in calculations: Not physical If hypothesize a fourth quark (‘charm’), problems resolved• Discovered 1973! In 1976 and 1977 Tau lepton and b (‘bottom’) quarks discovered Again, seemed to require an even number of quarks• Predict existence of ‘top quark’FermilabDzero ExperimentTop Quark Discovered, 1995Questions Explain why the weak force has a restricted distance