Observation of the Decay $K_L\to μ^+μ^- γγ$

Abstract

We have observed the decay $K_L\to μ^+μ^- γγ$ at the KTeV experiment at Fermilab. This decay presents a formidable background to the search for new physics in $K_L\toπ^0μ^+μ^-$. 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 ${\mathcal B}(K_L\to μ^+μ^- γγ$) $ = (10.4^{+7.5}_{-5.9} {\rm (stat)} \pm 0.7 {\rm (sys)})\times 10^{-9}$ with $m_{γγ} \geq 1 {\rm MeV/c}^2$, consistent with a QED calculation which predicts $(9.1\pm 0.8)\times 10^{-9}$.