$\gamma\gamma\to\pi^0\pi^0$ and $\eta\to\pi^0\gamma\gamma$ at $O(p^6)$ in the NJL model

We discuss the processes $\gamma\gamma\to\pi^0\pi^0$ and $\eta\to\pi^0 \gamma\gamma$ at $O(p^6)$ in the momentum expansion. The calculation involves tree--level, one--loop and two--loop diagrams of a chiral effective lagrangian which is obtained by a bosonization of the NJL model. The importance of integrating out meson resonances (reduction) is pointed out. Our final results for the total cross section of $\gamma\gamma\to\pi^0\pi^0$ are in good agreement with the experimental data of the Crystal Ball Collaboration. For the width of the $\eta\to\pi^0\gamma\gamma$ decay we obtain the value $0.11$ eV which has to be compared with the experimental value of $(0.84 \pm 0.18)$ eV. Alternatively, taking empirical parameters from a vector--meson--dominance model the prediction for the decay width is $0.35$ eV. We present a prediction for the differential decay probability as a function of $m_{\gamma\gamma}^2/m_\eta^2$.