Precision measurements of rare kaon decays

TL;DR

This paper presents precise measurements of rare kaon decays, including direct photon emission, interference effects, and branching fractions, using the extensive NA48/2 dataset, providing new insights into CP violation and decay dynamics.

Contribution

First observation of interference in direct photon emission in kaon decays and comprehensive measurements of branching fractions and form factors using large data samples.

Findings

Interference amplitude observed for the first time.

Branching fractions and form factors measured with high precision.

CP asymmetry in kaon decays quantified.

Abstract

We report the recent results on rare kaon decays from the NA48/2 experiment. The precise measurement of direct photon emission (DE) in the decay K+- -> pi+- pi0 gamma and its interference (INT), with the INT amplitude being observed for the first time, has been finalized. This study is based on the full NA48/2 data set with about 600k reconstructed K+- -> pi+- pi0 gamma decays which is factor of 30 larger than for previous experiments. Samples of about 7200 reconstructed K+- -> pi+- e+ e-, and more than 3000 K+- -> pi+- mu+ mu- events, with very small background contamination, have been collected. The latter is exceeding the total existing statistics by a factor of five. A precise measurement of the branching fractions and the form factors of the rare decays K+- -> pi+- l+ l- has been performed using different theoretical models. The CP violating asymmetry between K+ and K- in this channel is also measured.