New measurement of the K+- --> pi+-mu+mu- decay

TL;DR

This paper reports a precise measurement of the K±→π±μ+μ− decay's branching ratio, form factor, and asymmetries, providing new constraints on charge and lepton number violating processes.

Contribution

The study provides the first detailed measurement of the decay's form factor and asymmetries, improving understanding of rare kaon decays and testing Standard Model predictions.

Findings

Branching ratio BR=(9.62±0.25)×10⁻⁸

Form factor slope δ=3.11±0.57

Upper limits on charge asymmetry and lepton number violation

Abstract

A sample of 3120 $K^\pm\to\pi^\pm\mu^+\mu^-$ decay candidates with $(3.3\pm0.7)%$ background contamination has been collected by the NA48/2 experiment at the CERN SPS, allowing a detailed study of the decay properties. The branching ratio was measured to be ${\rm BR}=(9.62\pm0.25)\times 10^{-8}$. The form factor $W(z)$, where $z=(M_{\mu\mu}/M_K)^2$, was parameterized according to several models. In particular, the slope of the linear form factor $W(z)=W_0(1+\delta z)$ was measured to be $\delta=3.11\pm0.57$. Upper limits of $2.9\times 10^{-2}$ and $2.3\times 10^{-2}$ on possible charge asymmetry and forward-backward asymmetry were established at 90% CL. An upper limit ${\rm BR}(K^\pm\to\pi^\mp\mu^\pm\mu^\pm)<1.1\times 10^{-9}$ was established at 90% CL for the rate of the lepton number violating decay.