Measurement of the $K^+\toπ^+ν\barν$ Branching Ratio
Experiment E949 at Brookhaven National Laboratory studied the rare decay \$K^+\toπ^+ν\barν$ and other processes with an exposure of $1.77\times 10^{12}$ $K^+$'s. The data were analyzed using a blind analysis technique yielding one candidate event with an estimated background of $0.30\pm0.03$ events. Combining this result with the observation of two candidate events by the predecessor experiment E787 gave the branching ratio ${\calB}(K^+\toπ^+ν\barν)=(1.47^{+1.30}_{-0.89})\times 10^{-10}$, consistent with the Standard Model prediction of $(0.74\pm0.20)\times 10^{-10}$. This is a more detailed report of results previously published in Physical Review Letters.
hep-ex↗