First Observation and Study of the $K^\pm \to \pi^\pm \pi^0 e^+ e^-$ Decay

TL;DR

This paper reports the first observation of the rare decay $K^ o ^ ^0 e^+ e^-$, measuring its branching ratio and analyzing its kinematic and symmetry properties to test theoretical predictions.

Contribution

It provides the first experimental measurement of the decay's branching ratio and investigates its structure and asymmetries, confirming theoretical models.

Findings

Branching ratio measured as (4.24 ± 0.14)×10^{-6}

Evidence for structure-dependent contributions consistent with chiral perturbation theory

Evaluation of P- and CP-violating asymmetries

Abstract

The NA48/2 experiment at CERN reports the first observation of the $K^\pm \to \pi^\pm \pi^0 e^+ e^-$ decay from an exposure of $1.7 \times 10^{11}$ charged kaon decays recorded in $2003-2004$. A sample of 4919 candidates with 4.9% background contamination allows the determination of the branching ratio in the full kinematic region, BR($K^\pm \to \pi^\pm \pi^0 e^+ e^-)=(4.24 \pm 0.14)\times10^{-6}$. The study of the kinematic space shows evidence for a structure dependent contribution in agreement with predictions based on chiral perturbation theory. Several P- and CP-violating asymmetries are also evaluated.