In a hot, chirally symmetric phase, $\pi^0$ doesn't go into $2 \gamma$, but $\pi^0 \sigma$ does

Abstract

In a constituent quark model at nonzero temperature, the amplitude for $\pi^0 \rightarrow 2 \gamma$ vanishes in a chirally symmetric phase, while that for $\pi^0 \sigma \rightarrow 2 \gamma$ does not.