Observation of the Decay $K_L\to \mu^+\mu^- \gamma \gamma$

TL;DR

This paper reports the first observation of the decay $K_L o ul $ at Fermilab, measuring its branching ratio and confirming consistency with QED predictions, which is important for background understanding in new physics searches.

Contribution

First observation and measurement of the decay $K_L o ul $, providing experimental data consistent with theoretical QED calculations.

Findings

Branching ratio measured as (10.4^{+7.5}_{-5.9} ext{(stat)} ext{±} 0.7 ext{(sys)})×10^{-9}

Observed 4 events with expected background of 0.155 ± 0.081

Results align with QED prediction of (9.1 ± 0.8)×10^{-9}

Abstract

We have observed the decay $K_L\to \mu^+\mu^- \gamma \gamma$ at the KTeV experiment at Fermilab. This decay presents a formidable background to the search for new physics in $K_L\to\pi^0\mu^+\mu^-$. The 1997 data yielded a sample of 4 signal events, with an expected background of 0.155 $\pm$ 0.081 events. The branching ratio is ${\mathcal B}(K_L\to \mu^+\mu^- \gamma \gamma$) $ = (10.4^{+7.5}_{-5.9} {\rm (stat)} \pm 0.7 {\rm (sys)})\times 10^{-9}$ with $m_{\gamma\gamma} \geq 1 {\rm MeV/c}^2$, consistent with a QED calculation which predicts $(9.1\pm 0.8)\times 10^{-9}$.