Search for the lepton-family number violating decays KL→π0μ±e∓

Abstract We have carried out at Fermilab the first search for the lepton-family number violating decay K L → π 0 μ ± e ∓ . There were no events observed. By normalizing to K L → π + π − π 0 decays collected simultaneously, the 90% confidence level upper limit on 1 2 [ BR (K L →π 0 μ + e − )+ BR (K L →π 0 μ − e + )] was determined to be 3.1×10 −9 .