CPV tests with rare kaon decays

TL;DR

This paper reports on measurements of rare kaon decays, including branching ratios, form factors, and CP-violating asymmetries, using extensive data from the NA48/1 and NA48/2 experiments, significantly improving previous results.

Contribution

It provides new precise measurements of rare kaon decay modes, branching ratios, form factors, and CP asymmetries, with larger data samples and improved background suppression.

Findings

Measured K_S  e e- branching ratio and set limits on E1 emission.

Determined branching fractions and form factors for K  e- and K   mu mu- decays.

Found CP-violating asymmetry to be less than a few percent.

Abstract

The K_S \to pi+/- e+ e- decay mode has been investigated using the data collected in 2002 by the NA48/1 collaboration. With about 23k signal events and 59k K_L \to pi+ pi- pi0_D normalization decays, the K_S \to pi+ pi- e+ e- branching ratio was determined. This result is also used to set an upper limit on the presence of E1 direct emission in the decay amplitude. The CP-violating asymmetry has been also measured. We report on measurements of the rare decays K +/- \to pi+/- e+ e- and K+/- \to pi+/- mu+ mu- . The full NA48/2 data set was analyzed, leading to more than 7200 reconstructed events in the electronic and more than 3000 events in the muonic channel, the latter exceeding the total existing statistics by a factor of four. For both channels the selected events are almost background-free. From these events, we have determined the branching fraction and form factors of K+/- \to pi+/- e+ e- using different theoretical models. Our results improve the existing world averages significantly. In addition, we measured the CP violating asymmetry between K+ and K- in this channel to be less than a few percent.