Giles Barr

Abstract:

The KS → γγ decay rate has been measured with the NA48 detector using a high intensity short neutral beam from the CERN SPS. The measured branching ratio BR(KS → γγ) = (2.78 ± 0.06stat ± 0.04syst) x 10^-6, obtained from 7461 ± 172 KS → γγ events, is significantly higher than the O(p⁴) prediction of chiral perturbation theory. Using a KL beam the ratio Γ(KL → γγ)/Γ(KL → π⁰π⁰π⁰) = (2.81 ± 0.02syst) ×10^-3 has been measured. © 2002 Elsevier Science B.V. All rights reserved.

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.