Search for the decay K+→π+νν̄ in the momentum region Pπ<195 MeV/c

Abstract We have searched for the decay K + →π + ν ν in the kinematic region with pion momentum below the K + → π + π 0 peak. One event was observed, consistent with the background estimate of 0.73±0.18. This implies an upper limit on B(K + →π + ν ν ) −9 (90% C.L.), consistent with the recently measured branching ratio of 1.57 +1.75 −0.82 ×10 −10 , obtained using the standard model spectrum and the kinematic region above the K + → π + π 0 peak. The same data were used to search for K + → π + X 0 , where X 0 is a weakly interacting neutral particle or system of particles with 150 M X 0 c 2 .