Investigation of K_L, K_S --> pi+ pi- e+ e- decays

The K_L --> pi+ pi- e+ e− and K_S --> pi+ pi- 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 K_L --> pi+ pi- e+ e− candidates has been observed with an expected background level of 36.9 events, yielding the branching ratio measurement BR(K_L --> pi+ pi- e+ e−) = (3.08±0.20)×10−7. The distribution of events in the sin(phi) cos(phi) variable, where phi is the angle between the pi+pi- and the e+e− decay planes in the kaon centre of mass, is found to exhibit a large CP-violating asymmetry with the value A_{phi} = (14.2 ± 3.6)%. For the K_S --> pi+ pi- 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(K_S --> pi+ pi- e+ e-) = (4.71 ± 0.32) × 10−5. The combined value of this measurement with the published 1998 result is BR(K_S --> pi+ pi- 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 K_L --> pi+ pi- e+ e- direct emission process as well as the value of the K0 charge radius have been extracted from the data.