Anomalous mesonic interactions near a chiral phase transition
  Using constituent quarks coupled to a linear sigma model at nonzero temperature, I show that many anomalous mesonic amplitudes, such as $\pi^0 \rightarrow \gamma \gamma$, vanish in a chirally symmetric phase. Processes which are allowed, such as $\pi^0 \sigma \rightarrow \gamma \gamma$, are computed to leading order in a loop expansion. 
Robert D. Pisarski