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f$_{π}$ in equation (1.35) is a measure of the extent of the chiral symmetry breaking. If there is no chiral symmetry breaking its value is zero. In QCD it can be determined from measuring the decay rate of π 0 particles [24]. It is found to be 92 MeV. 

## Meson decay constants 

It is not just the pion that has a decay constant associated with it. All pseudoscalar mesons, P, have a decay constant F$_{P}$ , defined by 

And the vector meson decay constants ¶ are defined as: 

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where p and λ are the momentum and polarization state of the vector meson V ( p ; λ ). ϵ$_{µ}$ is the corresponding polarization vector. 

Experimentally, F$_{P}$ can be measured from leptonic decays of the ap- propriate meson [24]. F$_{V}$ can be measured from the decays of tau lep- 

