Measurement of theK+→π+νν¯branching ratio

Experiment E949 at Brookhaven National Laboratory studied the rare decay $K^{+}\to {\pi }^{+}\nu \overline{\nu }$ 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 $0.30\pm 0.03$ events. Combining this result with the observation of two candidate events by the predecessor experiment E787 gave the branching ratio $\mathcal{B}(K^{+}\to {\pi }^{+}\nu \overline{\nu })=({1.47}_{-0.89}^{+1.30})\times {10}^{-10}$, consistent with the standard model prediction of $(0.74\pm 0.20)\times {10}^{-10}$. This is a more detailed report of results previously published [V. V. Anisimovsky et al., Phys. Rev. Lett. 93, 031801 (2004)].