Search for the decaysK+→π+νν¯andK+→π+X0for150<MX0<250MeV/c2

A search for the decays ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}\ensuremath{\nu}\overline{\ensuremath{\nu}}$ and ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{X}^{0}$, where ${X}^{0}$ is any weakly interacting neutral particle or system of neutral particles with $150l{M}_{{X}^{0}}l250$ MeV/${\mathit{c}}^{2}$, was performed in the pion kinematic region below the ${K}_{\ensuremath{\pi}2}({K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{0})$ peak. An upper limit on $B({K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}\ensuremath{\nu}\overline{\ensuremath{\nu}})$ is set at 1.7 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}8}$ (90% C.L.) and $B({K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{X}^{0})$ is restricted to the ${10}^{\ensuremath{-}7}$ level or lower. Limits are also set for the production of ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}\ensuremath{\nu}\overline{\ensuremath{\nu}}$ through scalar or tensor interactions at 1.8 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}8}$ and 1.0 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}8}$ (90% C.L.), respectively.