$\eta \to \pi^0 \gamma \gamma$ to ${\cal{O}}(p)^6$ in Chiral Perturbation Theory

Abstract

The decay $\eta \to \pi^0 \gamma \gamma$ is discussed in the framework of SU(3) chiral perturbation theory. The process is dominated by the ${\cal{O}}(p^6)$ in the momentum expansion where tree-level amplitudes from the effective Lagrangian ${\cal{L}}^6$ enter together with one--loop contributions from ${ \cal{L}}^4$ and two--loop contributions from ${\cal{L}}^2$. We estimate the 6 independent ${\cal{L}}^6$ coupling constants by resonance saturation consistent with the pion production process $\gamma \gamma \to \pi^0 \pi^0$ and calculate the pion--loop part of the one- and two--loop amplitude. Predictions for the total rate and spectrum of $\eta \to \pi^0 \gamma \gamma$ are given together with a discussion of the uncertainties involved.