QCD

Why Colour?

1. Quark Wave Function of Baryons
   
2. Electron Positron Annihilation into Hadrons
   
3. tau decays
   
4. pi0 decay
   The dominant decay of a neutral pion is the electromagnetic decay in two photons: BF = 98.8 percent.
   
   Sideremark: Since the decay into three photons is not observed and electromagnetic interactions conserve charge conjugation C one concludes that the neutral pion is an eigenstate of C with eigenvalue +1.
   
   According to the quark model the flavor state of the pion in terms of quark states is given by The leading order Feyman diagrams for the pion decay into two photons due to quark antiquark annihilation is given by The blob means that the quark-antiquark pair is in a bound state.
   
   The matrix element is proportional to More precisely, the matrix element is given by where f_π is the pion decay constant which is a measure of the probability that the quark and antiquark are found at the origin. The pion decay constant can be measured in the leptonic decay of charged pions
   
   The decay rate (width) reads The experimental value clearly supports N_c=3.