First Evidence for the DecayKL→e+e−μ+μ−

We present the first evidence for the decay {ital K}{sub {ital L}}{r_arrow}{ital e}{sup +}{ital e}{sup {minus}}{mu}{sup +}{mu}{sup {minus}} based on the observation of one event with an estimated background of 0.067{sub {minus}0.025}{sup +0.057} event. We determine the branching ratio to be {ital B}({ital K}{sub {ital L}}{r_arrow}{ital e}{sup +}{ital e}{sup {minus}}{mu}{sup +}{mu}{sup {minus}})=(2.9{sub {minus}2.4}{sup +6.7}){times}10{sup {minus}9}. In addition, we set a 90{percent} confidence upper limit on the combined branching ratio for the lepton flavor violating decays {ital K}{sub {ital L}}{r_arrow}{ital e}{sup {minus_plus}}{ital e}{sup {minus_plus}}{mu}{sup {plus_minus}}{mu}{sup {plus_minus}} to be {ital B}({ital K}{sub {ital L}}{r_arrow}{ital e}{sup {minus_plus}}{ital e}{sup {minus_plus}}{mu}{sup {plus_minus}}{mu}{sup {plus_minus}}){lt}6.1{times}10{sup {minus}9} assuming a uniform phase space distribution. {copyright} {ital 1996 The American Physical Society.}