a Search for Effective Strangeness-Changing Neutral Currents in Neutral Longlived K-Meson Decays.

Results from AGS (Alternating Gradient Synchroton) experiment # 780 at Brookhaven National Laboratory are reported for the decay modes, K_sp{L }{0} to mu^+ + mu ^- and K_sp{L}{0 } to pi^{0} + e^+ + e^-. The detector consisted of a charged particle spectrometer of four mini-drift chambers, two upstream and two downstream of an analyzing magnet, a hydrogen Cherenkov counter and lead glass array for electron identification, and a muon filter and range stack consisting of a series of iron slabs followed by arrays of scintillator counters. Several additional counter arrays provided trigger level particle identification and event topology. Di-lepton triggers were accepted for mu + e, e + e and mu + mu . Two charged pion triggers were accepted for normalization to the CP-violating decay K_sp{L} {0} to pi^+ + pi^ -, and to K_sp{L}{0 } to pi^{0} + pi ^+ + pi^-. Two events consistent with the decay K _sp{L}{0} to mu^+ + mu^- were observed with a single event sensitivity of 4.6 times 10 ^{-9}. This result is consistent with the established branching ratio of (9.1 +/- 1.9) times 10^{-9}. While the sensitivity achieved in experiment # 780 did not allow for an improved measurement of this decay mode, the result is important as a check of the sensitivity established using the K _sp{L}{0} to pi^+ + pi^- decays. One event consistent with K_sp{L}{0} to pi^{0} + e^+ + e^ - was observed with a single event sensitivity of 2 times 10^{ -7} for the observed lepton pair mass of 328 MeV/c^2. If a phase space distribution is assumed, the single event sensitivity to this mode was 1 times 10^{ -6}. These rates for K_sp{L }{0} to pi^{0} + e^+ + e^- are much larger than those predicted in the Standard Model of weak interactions; this result, if confirmed, would be evidence for new physics.