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

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.