C. Arroyo

New limit on muon and electron lepton number violation from K0(L) ---> mu+- e-+ decay

The most sensitive experiment to date to search for the muon and electron lepton number violating decay

First observation of the rare decay mode K0(L) ---> e+ e-

K^0_L \to \mu^\pm e^\mp

A straw drift chamber spectrometer for studies of rare kaon decays

has detected no events consistent with this process. Based on this result, the 90% confidence level upper limit on the branching fraction is

New Limit on Muon and Electron Lepton Number Violation from K{sup 0}{sub L} {r_arrow} {mu}{sup {plus_minus} }e{sup {minus_plus}} Decay

B(K^0_L \to \mu^\pm e^\mp) < 4.7\times10^{-12}

First observations of separated atmospheric vu and vu events in the MINOS detector

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Improved Branching Ratio Measurement for the Decay K°{sub L} μ{sup +} μ⁻

In an experiment designed to search for and study very rare two-body decay modes of the K-long, we have observed four examples of the decay K-long -> 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} X 10^{-12}, consistent with recent theoretical predictions. This result represents by far the smallest branching fraction yet measured in particle physics.

New Limit on Muon and Electron Lepton Number Violation from K°{sub L} μ{sup ± }e{sup } Decay

We describe the design, construction, readout, tests, and performance of planar drift chambers, based on 5-mm-diameter copperized Mylar and Kapton straws, used in an experimental search for rare kaon decays. The experiment took place in the high-intensity neutral beam at the Alternating Gradient Synchrotron of Brookhaven National Laboratory, using a neutral beam stop, two analyzing dipoles, and redundant particle identification to remove backgrounds.