On Z → γγ decay and cancellation of axial anomaly in Z → γγ transition amplitude for massive fermions

Z → γγ decay amplitude is considered and proven to be zero due to properties of polarization vectors and Bose statistics. Triangular diagrams for a pseudoscalar → γγ and Z → γγ processes with massive fermions in the loop are explicitely calculated. In the Standard Model axial anomaly vanishes in the sum of these diagrams as Z boson is mixed with one of the Goldstone bosons.