Why Colour?

There are several strong evidences why quarks should carry - besides the electric charge - an additional internal degree of freedom: the colour charge.
  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 Nc colour charges the prediction is
    The measured values are
    which strongly supports Nc=3.
  4. pi0 decay