Search for the DecayK+→π+νν¯

An upper limit on the branching ratio for the decay $K^{+}\to {\pi }^{+}\nu \overline{\nu }$ is set at 7.5 × ${10}^{-9\mathrm{}}$ (90% C.L.) using the kinematic region above the $K_{\pi 2}(K^{+}\to {\pi }^{+}{\pi }^0)$ peak. When combined with our result from below the $K_{\pi 2}$ peak, we obtain the limit 5.2 × ${10}^{-9\mathrm{}}$. An upper limit of 1.7 × ${10}^{-9\mathrm{}}$ is also found on the branching ratio for decays of the form $K^{+}\to {\pi }^{+}{X}^0$ where $X^0$ is any massless, weakly interacting neutral particle. Limits are also set for cases where $M_{X^0}>0\mathrm{}$.