Search For The Decay K+->Pi(+) Nu(Nu)Over-Bar In The Momentum Region P-Pi < 195 Mev/C

We have searched for the decay K+ --> pi(+)nunu in the kinematic region with pion momentum below the K+ --> pi(+)pi(0) peak. One event was observed, consistent with the background estimate of 0.73 +/- 0.18. This implies an upper limit on B(K+ --> pi(+)nunu) < 4.2 x 10(-9) (90% C.L.), consistent with the recently measured branching ratio of (1.57(-0.82)(+1.75)) x 10(-10), obtained using the standard model spectrum and the kinematic region above the K+ --> pi(+)pi(0) peak. The same data were used to search for K+ --> pi(+)X(0), where X-0 is a weakly interacting neutral particle or system of particles with 150 < M-X0 < 250 MeV/c(2). (C) 2002 Elsevier Science B.V. All rights reserved.