Precision measurement of the ratio BR(KS → π+π-e+e-/ BR(KL →π+π-π0(D))

The KS→π+π−e+e− decay mode was investigated using the data collected in 2002 by the NA48/1 Collaboration. With about 23 k KS→π+π−e+e− events and 59 k normalization decays, the KS→π+π−e+e− branching ratio relative to the one was determined to be . This result was used to set the upper limit |gE1/gBR|<3.0 at 90% CL on the presence, in the decay amplitude, of an E1 direct emission (gE1) term relative to the dominant inner bremsstrahlung (gBR) term. The CP-violating asymmetry in the distribution of KS→π+π−e+e− events, where is the angle between the π+π− and the e+e− decay planes in the kaon centre of mass, was found to be , consistent with zero. These results are in good agreement with a description of the KS→π+π−e+e− decay amplitude dominated by the CP-even inner bremsstrahlung process.