Precise measurement of the K+- --> pi+-e+e- decay

TL;DR

This paper reports a precise measurement of the decay properties of the rare K+- --> pi+- e+e- decay, including branching ratio, form factor shape, and CP asymmetry limits, based on a large data sample from the NA48/2 experiment.

Contribution

The study provides the most precise measurement to date of the decay's branching ratio, form factor slope, and CP asymmetry limits for the K+- --> pi+- e+e- decay.

Findings

Branching ratio measured as (3.11 ± 0.12) × 10^{-7}.

Form factor slope δ determined to be 2.32 ± 0.18.

CP violation asymmetry upper limit set at 2.1 × 10^{-2} at 90% CL.

Abstract

A sample of 7253 $K^\pm\to\pi^\pm e^+e^-(\gamma)$ decay candidates with 1.0% background contamination has been collected by the NA48/2 experiment at the CERN SPS, allowing a precise measurement of the decay properties. The branching ratio in the full kinematic range was measured to be ${\rm BR}=(3.11\pm0.12)\times 10^{-7}$, where the uncertainty includes also the model dependence. The shape of the form factor $W(z)$, where $z=(M_{ee}/M_K)^2$, was parameterized according to several models, and, in particular, the slope $\delta$ of the linear form factor $W(z)=W_0(1+\delta z)$ was determined to be $\delta=2.32\pm0.18$. A possible CP violating asymmetry of $K^+$ and $K^-$ decay widths was investigated, and a conservative upper limit of $2.1\times 10^{-2}$ at 90% CL was established.