An upper limit on the branching ratio for the decay $K^{+}\to {\pi }^{+}\nu \overline{\nu }$ is set at $2.4\times {10}^{-9\mathrm{}}$ at the 90% C.L. using pions in the kinematic region $214\mathrm{MeV}/c<\mathrm{}{P}_{\pi }<231\mathrm{}\mathrm{MeV}/c$. An upper limit of $5.2\times {10}^{-10\mathrm{}}$ is found on the branching ratio for decays $K^{+}\to {\pi }^{+}{X}^0$, where $X^0$ is any massless, weakly interacting neutral particle. Limits are also set for cases where $M_{X^0}>0\mathrm{}$.

• Received 18 October 1995

DOI:https://doi.org/10.1103/PhysRevLett.76.1421