Measurement of the KAON+ Going to PION+ MUON+ - Branching Ratio

The search for the rm K^+ to pi^+mu^+mu^ - decay in the 1989-1991 data by Experiment 787 at Brookhaven National Laboratory is reported. This decay is suppressed to first order in the Standard Model because of the GIM mechanism, and it can only proceed through the exchange of a least two electroweak bosons. Comparison of this and other related rare kaon decay rates with the predictions of chiral perturbation theory can be used to constrain the theory's empirical parameters. The analysis is aimed at extracting a peak in the total kinetic energy distribution corresponding to the decay's Q-value of 142.76 MeV. A clear signal with a low level of background is observed. A fit to the energy spectrum yields 196.0 +/- 16.7 K_{pimu mu} events. This results in a branching ratio of B R(K^+topi^+ mu^+mu^-) = (4.98 +/- 0.43_{stat}+/- 0.59_ {sys})times 10^{-8}.