Investigation of KL,S → π+π -e+e- decays

The KL → π+ π− e+ e− and KS → π+ π− e+ e− decay modes have been studied in detail using the NA48 detector at the CERN SPS. Based on the data collected during the 1998 and 1999 run periods, a sample of 1162 KL → π+ π− e+ e− candidates has been observed with an expected background level of 36.9 events, yielding the branching ratio measurement BR(KL → π+ π− e+ e−) = (3.08 ± 0.20) × 10^−7 . The distribution of events in the sinφ cosφ variable, where φ is the angle between the π+ π− and the e+ e− decay planes in the kaon centre of mass, is found to exhibit a large CP-violating asymmetry with the value Aφ = (14.2 ± 3.6)%. For the KS → π+ π− e+ e− decay channel, 621 candidates have been identified in the 1999 data sample with an estimated background contribution of 0.7 event. The corresponding branching ratio has been determined to be BR(KS → π+ π− e+ e−) = (4.71 ± 0.32) × 10^−5 . The combined value of this measurement with the published 1998 result is BR(KS → π+ π− e+ e−) = (4.69 ± 0.30) × 10^−5 . No asymmetry is observed in this decay mode. Our results are in good agreement with theoretical predictions based on a phenomenological description of radiative kaon decays. The form factor parameters a1/a2 and gM1 in the KL → π+ π− e+ e− direct emission process as well as the value of the K0 charge radius have been extracted from the data.