Further Evidence for the Decay ${\mathit{K}}^{+}\to {\pi }^{+}\nu \overline{\nu }$

Additional evidence for the rare kaon decay $K^{+}\to {\pi }^{+}\nu \overline{\nu }$ has been found in a new data set with comparable sensitivity to the previously reported result. One new event was observed in the pion momentum region examined, $211<P<\mathrm{}229\mathrm{}\mathrm{MeV}/c$, bringing the total for the combined data set to two. Including all data taken, the backgrounds were estimated to contribute $0.15\pm 0.05$ events. The branching ratio is $B(K^{+}\to {\pi }^{+}\nu \overline{\nu }){=1.57\mathrm{}}_{-0.82\mathrm{}}^{+1.75\mathrm{}}\times {10}^{-10\mathrm{}}$.