Observation of the decay KS→π+π−e+e−

Abstract We present the first observation of the decay K S → π + π − e + e − based upon the data collected in 1998 by the NA48 experiment at CERN. We have identified a clean sample of 56 events with negligible background contamination. Using K L → π + π − π 0 D decays as normalization sample, the branching ratio is measured to be BR( K S → π + π − e + e − )=[4.5±0.7(stat)±0.4(syst)]×10 −5 . This result is in good agreement with the theoretical expectations from the mechanism of inner bremsstrahlung.