Recent results of BNL E791: 'Study of very rare KL0 decays'

The primary goal of BNL E791 is to search for the decay KL0→μe (nonconservation of separate lepton number) with a sensitivity of 10-11 to 10-12. The experiment also measures the branching ratio for the decay KL0→μμ, which was determined by previous experiments to be B(KL0→μμ)=(9.1±1.9)x10-9, and searches for the decays KL0→ee and KL0→π0ee, which are expected in the Standard Model with branching ratios of about 10-12 and 10-11. The data presented here were taken in the Spring 1988 run, and give a preliminary upper limit on the branching ratio B(KL0→μe)<3x10-10 (90% CL). About 75 KL0→μμ events were collected, three times the previous total world sample. No candidates for the decay KL0→ee were seen. We gained confidence in our ability to handle high beam intensity, up to 1x1013 protons per pulse, and to reject known backgrounds to the 10-11 level. (orig.)