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 $\mathcal{B}(K_L\to e^{\pm }{e}^{\pm }{\mu }^{\mp }{\mu }^{\mp })\le 1.23\times 10^{-10}$, assuming a uniform phase space distribution.