Branching Ratio Measurement of the decay K_L -> e+ e- mu+ mu-

TL;DR

This paper reports a precise measurement of the branching ratio for the decay K_L -> e+ e- mu+ mu-, finds no evidence of CP violation, and sets upper limits on lepton flavor violating decays, contributing to understanding rare kaon decays.

Contribution

The study provides the first measurement of the branching ratio for K_L -> e+ e- mu+ mu- with negligible background and establishes upper limits on lepton flavor violation in this decay channel.

Findings

Branching ratio measured as (2.62 ± 0.40 ± 0.17)×10^{-9}

No evidence for CP violation observed

Upper limit set on lepton flavor violating decays at 1.23×10^{-10}

Abstract

We have collected a 43 event sample of the decay $K_L\to e^+e^-\mu^+\mu^-$ with negligible backgrounds and measured its branching ratio to be $(2.62 \pm 0.40 \pm 0.17)\times 10^{-9} $. We see no evidence for CP violation in this decay. In addition, we set the 90% confidence upper limit on the combined branching ratios for the lepton flavor violating decays $K_L\to e^{\pm}e^{\pm}\mu^{\mp}\mu^{\mp}$ at ${\mathcal B}(K_L\to e^{\pm}e^{\pm}\mu^{\mp}\mu^{\mp}) \leq 1.23\times10^{-10}$, assuming a uniform phase space distribution.