Measurement of the $K^{+}\to {\pi }^{+}\nu \bar{\nu }$ Branching Ratio

Experiment E949 at Brookhaven National Laboratory studied the rare decay $K^+\to\pi^+\nu\bar{\nu}$andotherprocesseswithanexposureof$1.77\times 10^{12}$ $K^{+}$'s. The data were analyzed using a blind analysis technique yielding one candidate event with an estimated background of $0.30\pm 0.03$ events. Combining this result with the observation of two candidate events by the predecessor experiment E787 gave the branching ratio $\text{calB}(K^{+}\to {\pi }^{+}\nu \bar{\nu })=(1.47_{-0.89}^{+1.30})\times {10}^{-10}$, 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.