Precise measurements of the $K_{S}\to\gamma\gamma$ and $K_{L}\to\gamma\gamma$ decay rates

The $K_{S} \to \gamma\gamma$ decay rate has been measured with the NA48 detector using a high intensity short neutral beam from the CERN SPS. The measured branching ratio BR($K_{S}) \to \gamma\gamma = (2.78 \pm 0.06_{stat} \pm 0.04_{syst}) \times 10^{-6}$, obtained from 7461$\pm$172 $K_{S} \to \gamma\gamma$ events, is significantly higher than the $O(p^{4})$ prediction of Chiral Perturbation Theory. Using a $K_{L}$ beam the ratio $\Gamma(K_{L} \to \gamma\gamma)/ \Gamma(K_{L} \to \pi^{0}\pi^{0}\pi^{0}) = (2.81 \pm 0.01_{stat} \pm 0.02_{syst}) \times 10^{-3}$ has been measured.