G. M. Irwin

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

Properties of bottom quark jets in e+e- annihilation at 29 GeV☆

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

Search for the weak decay of an H dibaryon.

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

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

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

Measurement of the Cabibbo Suppressed Decays

.

\tau \to K

Abstract A sample of events enriched with b b production is obtained with the DELCO detector at PEP and is used to measure the mean multiplicity and spectra of charged particles in bottom-quark jets. Bottom-quark jets are found to be significantly softer and more spherical than jets in general hadronic events.

\nu_\tau

We have searched for a neutral {ital H} dibaryon decaying via {ital H}{r_arrow}{Lambda}{ital n} and {ital H}{r_arrow}{Sigma}{sup 0}{ital n}. Our search has yielded two candidate events from which we set an upper limit on the {ital H} production cross section. Normalizing to the inclusive {Lambda} production cross section, we find ({ital d}{sigma}{sub {ital H}}/{ital d}{Omega})/({ital d}{sigma}{sub {Lambda}}/{ital d}{Omega}){lt}6.3{times}10{sup {minus}6} at 90{percent} C.L., for an {ital H} of mass {approx_equal}2.15 GeV/{ital c}{sup 2}. {copyright} {ital 1996 The American Physical Society.}

and

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.

\tau \to K

The branching fractions for the Cabibbo-suppressed tau decays are measured using the DELCO detector at PEP. The results, R(tau..-->..K..nu../sub tau/) = (0.59 +- 0.18)% and R(tau..-->..K..nu../sub tau/+n..pi../sup 0/) = (1.71 +- 0.2 9)% including n = 0, are in agreement with the standard theory of sequential leptons and with tau-..mu..-e universality.

\nu_\tau n \pi^0

This paper reports the details of an experiment designed to detect the decay ..pi../sup 0/..-->..e/sup +/e/sup -/ and to measure its branching ratio. The experiment used a beam of 300-MeV/c ..pi../sup -/ mesons incident on a liquid-hydrogen target to produce neutral pions in the reaction ..pi../sup -/p..--> pi../sup 0/n. Electron-positron pairs were detected in a magnetic spectrometer by multiwire proportional chambers. A gas Cherenkov counter provided electron identification. The effective-mass distribution of e/sup +/e/sup -/ pairs was decomposed with the aid of a Monte Carlo simulation of the ..pi../sup 0/..-->..e/sup +/e/sup -/ signal and background processes. The result is 59 +- 21 events, which corresponds to GAMMA(..pi../sup 0/..-->..e/sup +/e/sup -/)/GAMMA(..pi../sup 0/..--> gamma gamma..) = (17 +- 6 +- 3) x 10/sup -8/, where the first error is due to statistics and the second is an estimate of systematic effects. The central value is almost four times the unitarity lower limit while existing calculations generally predict values no larger than twice the unitarity limit.We have measured the branching ratio for [ital K][sub [ital L]][sup 0][r arrow][mu][sup +][mu][sup [minus]] using our full data set obtained during running periods in 1988, 1989, and 1990. The total number of [mu][sup +][mu][sup [minus]] candidates after a background subtraction is 707, which represents the largest sample to date of this rare decay mode. Our result is [ital B]([ital K][sub [ital L]][sup 0][r arrow][mu][sup +][mu][sup [minus]])=(6.86[plus minus]0.37)[times]10[sup [minus]9], which is consistent with earlier results and very near the unitarity bound. The upper limit on [ital B]([ital K][sub [ital L]][sup 0][r arrow][mu][sup +][mu][sup [minus]]) is used to set an upper limit on the real part of the amplitude [ital scrA]([ital K][sub [ital L]][sup 0][r arrow][mu][sup +][mu][sup [minus]]); this is subsequently used to constrain the top quark mass [ital m][sub [ital t]] and the Wolfenstein parameter [rho] of the CKM matrix.