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

The decay rate of $\klpgg$ has been measured with the NA48 detector at the CERN SPS. A total of $2558$ $\klpgg$ candidates have been observed with a residual background of $3.2\%$. The branching ratio is determined to be $(1.36 \pm0.03_{(stat)} \pm0.03_{(syst)}\pm 0.03_{(norm)})\times10^{-6}$ 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 $\klpee$ decay. An upper limit for the $\klpgg$ decay rate in the two photon mass region $m_{\gamma\gamma} m_{\pi^0}$ is also give