Search for the decays K+→π+νν̄ and K+→π+X0 for 150<MX0<250 MeV/c2

A search for the decays K+→π+νν̄ and K+→π+X0, where X0 is any weakly interacting neutral particle or system of neutral particles with 150<MX0<250 MeV/c2, was performed in the pion kinematic region below the Kπ2 (K+→π+π0) peak. An upper limit on B(K+→π+νν̄) is set at 1.7 × 10-8 (90% C.L.) and B(K+→π+X0) is restricted to the 10-7 level or lower. Limits are also set for the production of K+→π+νν̄ through scalar or tensor interactions at 1.8 × 10-8 and 1.0 × 10-8 (90% C.L.), respectively.