Experimental Search for the Decay Long-Lived Neutral Kaon Going to Muonium

An experimental study of very rare decays of neutral kaons has been carried out at a new neutral beam line at the Alternating-Gradient Synchrotron at Brookhaven National Laboratory. One focus of this study was a search for the lepton-flavor violating decay K_sp{L }{0}tomu e. No candidate events indicating occurrence of this process were found. Measurement of the experimental sensitivity was performed by simultaneous observation of the decay K_sp{L} {0}topi^+pi^- . This yielded a limited at the 90% confidence level on the branching ratio B(K_sp{L }{0}tomu e) < 2.2 times 10^ {-10}. This limit represents a substantial improvement over previous limits on the rate for this process.