Search for the decay $K^{+}\to {\pi }^{+}\nu \bar{\nu }$ in the momentum region $P_{\pi }<195MeV/c$

Abstract

We have searched for the decay $K^{+}\to {\pi }^{+}\nu \bar{\nu }$ in the kinematic region with pion momentum below the $K^{+}\to {\pi }^{+}{\pi }^0$ peak. One event was observed, consistent with the background estimate of $0.73\pm 0.18$. This implies an upper limit on $B(K^{+}\to {\pi }^{+}\nu \bar{\nu })<4.2\times 10^{-9}$ (90% C.L.), consistent with the recently measured branching ratio of $(1.57_{-0.82}^{+1.75})\times 10^{-10}$, obtained using the standard model spectrum and the kinematic region above the $K^{+}\to {\pi }^{+}{\pi }^0$ peak. The same data were used to search for $K^{+}\to {\pi }^{+}{X}^0$, where $X^0$ is a weakly interacting neutral particle or system of particles with $150<M_{X^0}<250MeV/{\mathrm{c}}^2$.

4 pages, 2 figures