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) x 10-7. The distribution of events in the sinφcosφ 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) x 10-5. The combined value of this measurement with the published 1998 result is BR(KS→π+π-e+e-)=(4.69±0.30) x 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. (orig.)