Precise measurement of the decay KL → π 0 γγ

The decay rate of KL → π0 γγ has been measured with the NA48 detector at the CERN SPS. A total of 2558 KL π0γγ candidates have been observed with a residual background of 3.2%. The branching ratio is determined to be (1.36 ± 0.03(stat) ± 0.03(syst) ± 0.03(norm)) × 10-6 and the vector coupling constant av = -0.46 ± 0.03(stat) ± 0.04(syst). This result suggests that the CP-violation effects are dominating in the KL → π0e+e- decay. An upper limit for the KL → π0γγ decay rate in the two photon mass region mγγ < mπ0 is also given. © 2002 Published by Elsevier Science B.V.