The most sensitive experiment to date to search for the muon and electron lepton number violating decay K 0 L ! m 6 e 7 has detected no events consistent with this process. Based on this result, the 90% confidence level upper limit on the branching fraction is B͑K 0 L ! m 6 e 7 ͒ , 4.7 3 10 212 . [S0031-9007(98)07985-X]
We report results from Experiment 871, performed at the BNL AGS, of a measurement of the branching ratio K(0)(L)-->&mgr;(+)&mgr;(-) with respect to the CP-violating mode K(0)(L)-->pi(+)pi(-). This experiment detected over 6200 candidate &mgr;(+)&mgr;(-) events, a factor of 6 more than that seen in all previous measurements combined. The resulting branching ratio gamma(K(0)(L)-->&mgr;(+)&mgr;(-))/gamma(K(0)(L)-->pi(+)pi(-)) = (3. 474+/-0.057)x10(-6) leads to a branching fraction B(K(0)(L)-->&mgr;(+)&mgr;(-)) = (7.18+/-0.17)x10(-9), which is consistent with the current world average, and reduces the uncertainty in this decay mode by a factor of 3.
In an experiment designed to search for and study very rare two-body decay modes of the K 0 L , we have observed four examples of the decay K 0 L → e + e − , where the expected background is 0.17 ± 0.10 events. This observation translates into a branching fraction of 8.7+5.7 −4.1 × 10 −12 , consistent with recent theoretical predictions. This result represents by far the smallest branching fraction yet measured in particle physics.
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