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

An additional event near the upper kinematic limit for $K^{+}\to {\pi }^{+}\nu \overline{\nu }$ has been observed by experiment E949 at Brookhaven National Laboratory. Combining previously reported and new data, the branching ratio is $\mathcal{B}(K^{+}\to {\pi }^{+}\nu \overline{\nu })=\left({1.47}_{-0.89}^{+1.30}\right)\times {10}^{-10}$ based on three events observed in the pion momentum region $211<P<229\mathrm{M}\mathrm{e}\mathrm{V}/c$. At the measured central value of the branching ratio, the additional event had a signal-to-background ratio of 0.9.

Figure 1

Range versus energy distribution of events passing all other cuts of the final sample. The circles represent E787 data and the triangles represent E949 data. The group of events around $E=108\mathrm{M}\mathrm{e}\mathrm{V}$ was due to the $K_{\pi 2}$ background. The simulated distribution of events from $K^{+}\to {\pi }^{+}\nu \overline{\nu }$ decay is indicated by dots. The solid line (dashed line) box represents the signal region for E949 (E787).