  $Z \to \gamma \gamma$ decay amplitude is considered and proven to be zero due
to properties of polarization vectors and Bose statistics. Triangular diagrams
for a pseudoscalar $\to \gamma \gamma$ and $Z \to \gamma \gamma$ 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.

High Energy Physics - Phenomenology