Investigation of K(L,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) x 10(-7). The distribution of events in the sinphicosphi 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) x 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) 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 a(1)/a(2) and (g) over tilde (M1) in the K-L --> pi(+)pi(-)e(+)e(-) direct emission process as well as the value of the K-0 charge radius have been extracted from the data