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

Abstract

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 $(1.36\pm 0.03_{(stat)}\pm 0.03_{(syst)}\pm 0.03_{(norm)})\times 10^{-6}$ and the vector coupling constant $a_v=-0.46\pm 0.03_{(stat)}\pm 0.04_{(syst)}$. 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 given.

12 pages, 7 figures and 2 tables. to be published in Phisics Letters B