  At zero temperature, in the chiral limit the amplitude for pi0 to decay into
two photons is directly related to the coefficient of the axial anomaly. At any
nonzero temperature, this direct relationship is lost: while the coefficient of
the axial anomaly is independent of temperature, in a thermal bath the
anomalous Ward identities do not uniquely constrain the amplitude for pi0 ->
gamma gamma. Explicit calculation shows that to lowest order about zero
temperature, this amplitude decreases.

High Energy Physics - Phenomenology;Nuclear Theory