## QCD

### Why Colour?

There are several strong evidences why quarks should carry - besides the electric charge -
an additional internal degree of freedom: the colour charge.
__Quark Wave Function of Baryons__

__Electron Positron Annihilation into Hadrons__

__tau decays__

__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.