QCD

Why Colour?

1. Quark Wave Function of Baryons
   
2. Electron Positron Annihilation into Hadrons
   
3. tau decays
   Tau leptons may decay weakly into pure leptonic final states or into hadronic final states plus a tau neutrino At tree level the Feynman diagrams for these decays look like Neglecting particle masses in the final state and assuming lepton universality (i.e. the weak coupling constant is the same for leptons and quarks) the rates for all these processes are equal.
   
   As a consequence, the leptonic branching fractions are predicted to be: If one assumes that quarks cary N_c colour charges the prediction is The measured values are which strongly supports N_c=3.
   
4. pi0 decay