Giles Barr

Abstract:

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(K S → 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 g̃M1 in the K L → π+π-e+e- direct emission process as well as the value of the K0 charge radius have been extracted from the data.

Abstract:

A measurement of the KS lifetime is presented using data recorded by the NA48 experiment at the CERN-SPS during 1998 and 1999. The KS lifetime is derived from the ratio of decay time distributions in simultaneous, collinear KS and KL beams, giving a result which is approximately independent of the detector acceptance and with reduced systematic errors. The result obtained is τS = (0.89598 ± 0.00048 ± 0.00051) × 10-10 s, where the first error is statistical and the second systematic. © 2002 Published by Elsevier Science B.V.