First Observation and Measurement of the Decay K+- -> pi+- e+ e- gamma

TL;DR

This paper reports the first observation and measurement of the rare decay K+- -> pi+- e+ e- gamma, providing a model-independent branching ratio and spectrum for comparison with theoretical predictions.

Contribution

First experimental observation and measurement of the decay K+- -> pi+- e+ e- gamma with a detailed branching ratio and spectrum analysis.

Findings

Branching ratio measured as (1.19 +- 0.12_stat +- 0.04_syst) x 10^-8.

120 candidate events observed with estimated background of 7.3 +- 1.7.

Spectrum allows comparison with Chiral Perturbation Theory predictions.

Abstract

Using the full data set of the NA48/2 experiment, the decay K+- -> pi+- e+ e- gamma is observed for the first time, selecting 120 candidates with 7.3 +- 1.7 estimated background events. With K+- -> pi+- pi0D as normalisation channel, the branching ratio is determined in a model-independent way to be Br(K+- -> pi+- e+ e- gamma, m_eegamma > 260 MeV/c^2) = (1.19 +- 0.12_stat +- 0.04_syst) x 10^-8. This measured value and the spectrum of the e+ e- gamma invariant mass allow a comparison with predictions of Chiral Perturbation Theory.