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Abstract

Experiment 787 at Brookhaven National Laboratory, is a sensitive search for the decay of a positively-charged kaon into a positively-charged pion, a neutrino, and an anti-neutrino: K+ —> π+vv. This extremely rare decay is expected to occur in the Standard Model with a branching ratio of (0.8 ± 0.3) x 10[superscript -10]. It also serves as a hunting ground for new physics. Data collected between 1995 and 1997 contains one event consistent with K+ —> π+vv, with background estimated to contribute 0.08 ± 0.02 events. Because this event survives the tightest data-selection cuts which further reduce the estimated background to 0.006 ± 0.002 events, it is likely due to K+ —> π+vv. 3.24 x 10[superscript 12] kaons were collected with an acceptance for K+ --> π+vv of (0.21 ± 0.02)%, resulting in a K+ --> π+vv branching ratio of 1.5[superscript +3.4, subscript -1.2] x 10[superscript -10] . The weak coupling of top to down quarks, parameterized by the Cabibbo-Kobayashi- Maskawa matrix element Vtd, is calculated to lie in the range 0.002 < |Vtd| < 0.04. The upper limit on the K+ --> π+f branching ratio, for massless f and for no events observed, is 1.1 x 10[superscript -10] at the 90% confidence level.