Measurement of the KL→e+e−e+e− decay rate

The decay rate of the long-lived neutral K meson into the e+e−e+e− final state has been measured with the NA48 detector at the CERN SPS. Using data collected in 1998 and 1999, a total of 200 events has been observed with negligible background. This observation corresponds to a branching ratio of Br(KL→e+e−e+e−)=(3.30±0.24stat±0.23syst±0.10norm)×10−8.