Abstract
The radiative decays η′(η) → γγ are discussed. The modifications of the conventional PCAC formulae due to the gluonic contribution to the flavour singlet axial anomaly are given. The decay constants satisfy a modified Dashen formula which generalises the Witten-Veneziano formula for the mass of the η′. It is shown how the topological susceptibility in QCD with massive, dynamical quarks may be extracted from measurements of η′(η) → γγ.
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