Search for the decay K+→π+νν̄

An upper limit on the branching ratio for the decay {ital K}{sup +}{r_arrow}{pi}{sup +}{nu}{nu} is set at 2.4{times}10{sup {minus}9} at the 90{percent} C.L. using pions in the kinematic region 214 MeV/{ital c}{lt}{ital P}{sub {pi}}{lt}231 MeV/ {ital c}. An upper limit of 5.2{times}10{sup {minus}10} is found on the branching ratio for decays {ital K}{sup +}{r_arrow}{pi}{sup +}{ital X}{sup 0}, where {ital X}{sup 0} is any massless, weakly interacting neutral particle. Limits are also set for cases where {ital M}{sub {ital X}{sup 0}}{approx_gt}0. {copyright} {ital 1996 The American Physical Society.}