$\eta \to \pi^0 \gamma \gamma$ decay in the three-flavor   Nambu-Jona-Lasinio model
  We study the $\eta \to \pi^0 \gamma \gamma$ decay via the quark-box diagram in the three-flavor Nambu--Jona-Lasinio model that includes the $U_A (1)$ breaking effect. We find that the $\eta$-meson mass, the $\eta \to \gamma \gamma$ decay width and the $\eta \to \pi^0 \gamma \gamma$ decay width are in good agreement with the experimental values when the $U_A (1)$ breaking is strong and the flavor $SU(3)$ singlet-octet mixing angle $\theta$ is about zero. The photon energy and the photon invariant mass spectra in $\eta \to \pi^0 \gamma \gamma$ are compared with those in the chiral perturbation theory. 
Y. Nemoto, M. Oka, M. Takizawa