Investigation of $K_\mathrm{L,S} \rightarrow \pi^+ \pi^{-} e^+ e^{-}$ decays

Abstract

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 → π + π − e + e − candidates has been observed with an expected background level of 36.9 events, yielding the branching ratio measurement BR(K L → 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 A φ =(14.2 ± 3.6)%. For the K S → 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(K S → 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 a 1 /a 2 andg M1 in the K L → π + π − e + e − direct emission process as well as the value of the K 0 charge radius have been extracted from the data.

Kaon electromagnetic form factors in dispersion theory

“…[26] are based on the spacelike data [21,22] only, such that the comparison illustrates the impact of the timelike data sets, as well as the dispersiontheoretical constraints on the isovector part. For the neutral kaon, constraints on the charge radius can be extracted from K L → π + π − e + e − [27,28] and electron scattering experiments [29][30][31], allowing for another cross check. 2.…”

Precise Determination of the Branching Ratio of the Neutral-Pion Dalitz Decay

“…An example is the K + → π + e + e − decay [2]: accurate knowledge of B(π 0 D ) would improve the precision on the rate measurement by 30 %, and the precision on the low-energy parameter a + [3] by 10 %. The uncertainty on B(π 0 D ) also dominates the precision on the K ± → π ± π 0 e + e − rate measurement [4], and is among the principal contributions to the uncertainties on the measured K L,S → π + π − e + e − rates [5]. In these circumstances, considering the improving precision on raredecay measurements, and the recent progress on the π 0form-factor measurement [6] and radiative corrections for the π 0 D decay [7], both a precision measurement of B(π 0 D ) and an updated theoretical evaluation of this quantity are becoming more important.…”