Experimental study of the rare decays K⁰_L → μe, K⁰_L → ee, K⁰_L → μμ, and K⁰_L → eeee

An experiment to search for the KL0→μe and KL0→ee decays and to measure the branching ratio of the KL0→μμ decay was performed at the KEK 12-GeV Proton Synchrotron. The single-event sensitivity of the present experiment for these channels was around 4×10-11. No event was observed in the fiducial region for the KL0→μe decay and one event, which could not be distinguished from the background, was observed in the fiducial region for the KL0→ee decay. The obtained upper limits on the branching ratios at 90% confidence level were B(KL0→μe)<9.4×10-11 and B(KL0→ee)<1.6×10-10. The branching ratio of the KL0→μμ decay was obtained to be [7.9±0.6 (stat ±0.2 (syst)]×10-9 based on 179 events in the fiducial region. Using 18 events with the effective mass Me+e->470 MeV /c2 in the e+e- sample for the KL0→ee decay search, the branching ratio of the KL0→e+e-e+e- decay was estimated to be [6±2 (stat) ±1 (syst)]×10-8 for Me+e->470 MeV/c2 and [7±3 (stat)±2(syst)]×10-8 for Me+e->480 MeV/c2.