AGS-787, BNL-Princeton-TRIUMF, began taking data in February 1988 on rare K decays into .pi. and non-interacting neutrals. Recent observations of B-/bar B/ mixing have sharpened the branching ratio prediction of the three generation standard model, (SM), to (approximately) 1-5 x 10−1° for .pi.+.nu./bar /nu//. The detector has 2.pi. acceptance for this decay and 4.pi. acceptance for background gammas. A very large window for viewing new physics exists between the SM value and the previous experimental limit of 1.4 x 10−7. Several other predicted rare K+ decays can be detected. The performance of the detector is described. 8 refs., 4 figs.

Results are reported from a Dalitz plot analysis of 17750 K/sup 0//sub e3/ candidates obtained in an experiment designed to study both the semileptonic and 3.pi. decay modes of the K/sup 0//sub L/ with high efficiency across the Dalitz plot.

The authors observed a new neutral kaon decay mode, K{sub L} {yields} {pi}{sup {+-}}e{sup {-+}}{bar {nu}}e{sup +}e{sup -} for the first time. Based on the 20225 events including 1018 {+-} 25 background events, they determined the branching ratio, B(K{sub L} {yields} {pi}{sup {+-}}e{sup {-+}}{bar {nu}}e{sup +}e{sup -}; M{sub e{sup +}e{sup -}}> 5 MeV/c{sup 2}, E*{sub e{sup +}e{sup -}}> 30 MeV) = (1.281 {+-} 0.041) x 10{sup -5}. This branching ratio agrees with a theoretical prediction based on the chiral perturbation theory ({chi}PT) calculation at {Omicron}(p{sup 4}). Most of the kinematical distributions agree with the {chi}PT {Omicron}(p{sup 4}) calculation. They also measured one of the low energy coupling constants for the {chi}PT {Omicron}(p{sup 4}), L{sub 9}{sup r} = (8.0 {+-} 1.6) x 10{sup -3}. The M{sub e{sup +}e{sup -}} distribution below 100 MeV showed a 3 {sigma} deviation from the {chi}PT {Omicron}(p{sup 4}) calculation. This requires further studies in theory and experiments.