Search for the rare charged decay kaon->pion neutrino antineutrino

An experimental search for the flavour-changing weak neutral current decay K⁺ → π⁺ vv was performed in the π+ kinematic energy range 60 < Tπ+ < 100 MeV. The Standard Model theo- retical prediction for the branching ratio is within the bounds (0.5—4.5) x 10⁻¹⁰. An observation at a rate significantly in excess of the prediction would be an indicator of new physics. The experiment was also sensitive to decays of the type K⁺ → π⁺X⁰, where X° is a weakly interacting massive neutral particle with mass in the range 150 < Mxo < 260 M.2eV/c² The search was carried out at Brookhaven National Laboratory, using an intense beam of positively charged kaons. A specialized detector selected candidate K⁺ → π⁺vv decays among a large background of other processes by identifying K⁺ decays at rest, measuring the momentum, kinetic energy and range of the π⁺ , recording its decay sequence for positive identification and vetoing photons from background processes. Data were collected in two different periods in 1989 and 1991. After analysis of the 1989 data set, no events were observed, allowing a 90% confidence level upper limit of 1.7 x 10⁻⁸ to be set for the branching ratio. This is a factor of 55 improvement over the previous experiment using the same region of phase space. Studies performed after the analysis was completed led to an estimated expected background of less than one event. In the 1991 data set, atotalof4 events were observed, with an estimated background of less than two events. The studies leading to this estimate were performed before the final analysis was carried out, resulting in reduced bias. However, uncertainties in the background estimate precluded the identification of the source of the events. Therefore, an upper limit of 5.0 x 10⁻⁸ was set for the branching ratio, based on the observation of 4 events.Science, Faculty ofPhysics and Astronomy, Department ofGraduat