Radiative $\eta'$ decays, the topological susceptibility and the Witten-Veneziano mass formula

The formulae describing the radiative decays eta '( eta ) to gamma gamma in QCD beyond the chiral limit are derived. The modifications of the conventional PCAC formulae due to the gluonic contribution to the axial anomaly in the flavour singlet channel are precisely described. The decay constants are found to satisfy a modified Dashen formula which generalises the Witten-Veneziano formula for the mass of the eta '. Combining these results, it is shown how the topological susceptibility in QCD with massive, dynamical quarks may be extracted from measurements of eta '( eta ) to gamma gamma. (35 refs).