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Lepton and Baryon Conservation

Lepton and Baryon Conservation

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Lepton and Baryon Conservation

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  1. Lepton and Baryon Conservation • Strong and EM conserve particle type. Weak can change but always lepton->lepton or quark->quark • So number of quarks (#quarks-#antiquarks) conserved. Sometimes called baryon conservation B. • Number of each type (e,mu,tau) conserved L conservation • Can always create particle-antiparticle pair • But universe breaks B,L conservation as there is more matter than antimatter • At small time after big bang #baryons = #antibaryons = #leptons = #antileptons (modulo spin/color/etc) • Now baryon/photon ratio 10-10 P461 - particles II

  2. Hadron production + Decay • Allowed production channels are simply quark counting • Can make/destroy quark-antiquark pairs with the total “flavor” (upness = #up-#antiup, downness, etc) staying the same • All decays allowed by mass conservation occur quickly (<10-21 sec) with a few decaying by EM with lifetimes of . 10-16 sec) Those forbidden are long-lived and decay weakly and do not conserve flavor. P461 - particles II

  3. Hadrons and QCD • Hadrons are made from quarks bound together by gluons • EM force QuantumElectroDynamics QED strong is QuantumChromoDynamics QCD • Strong force “color” is equivalent to electric charge except three different (identical) charges red-green-blue. Each type of quark has electric charge (2/3 up -1/3 down, etc) and either r g b (or antired, antiblue, antigreen) color charge • Unlike charge=0 photon, gluons can have color charge. 8 such charges(like blue-antigreen) combos, 2 are colorless. Gluon exchange usually color exchange. Can have gluon-gluon interaction P461 - particles II

  4. Pions • Use as strong interaction example • Produce in strong interactions • Measure pion spin. Mirror reactions have same matrix element but different phase space/kinematics term. “easy” part of phase space is just the 2s+1 spin degeneracy term • Find S=0 for pions P461 - particles II

  5. More Pions • Useful to think of pions as I=1isospin triplet and p,n is I=1/2 doublet (from quark plots) • Look at reactions: • p p -> d pi+ Total I ½ ½ 0 1 1 Iz ½ ½ 0 1 1 p n -> d pi0 Total I ½ ½ 0 1 0 or 1 Iz ½ - ½ 0 0 0 • in the past we combined 2 spin ½ states to form S=0 or 1 P461 - particles II

  6. More Pions • Reverse this and say eigentstate |p,n> is combination of I=1 and I=0 • reactions: • then take the “dot product” between |p,n> and |d,pi0> brings in a 1/sqrt(2) (the Clebsch-Gordon coefficient) • Square to get A/B cross section ratio of 1/2 P461 - particles II