Precise measurement of the decay $K_{L} \to \pi^{0}\gamma\gamma$

The decay rate of $K_{L} \to \pi^{0}\gamma\gamma$ has been measured with the NA48 detector at the CERN SPS. A total of 2558 $K_{L} \to \pi^{0}\gamma\gamma$ candidates have been observed with a residual background of 3.2%. The branching ratio is determined to be $(1.36 \pm 0.03_{(stat)} \pm 0.03_{(syst)} \pm 0.03_{(norm)}) \times10^{-6}$ and the vector coupling constant $a_v = -0.46 \pm0.03_{(stat)} \pm0.04_{(syst)}$. This result suggests that the CP-violation effects are dominating in the $K_{L} \to \pi^{0}e^{+}e^{-}$ decay. An upper limit for the $K_{L} \to \pi^{0}\gamma\gamma$ decay rate in the two photon mass region $m_{\gamma\gamma}<m_{\pi^0}$ is also given.