Experiment E949 at Brookhaven National Laboratory studied the rare decay K ! and other processes with an exposure of 1:77 10 12 K 's. The data were analyzed using a blind analysis technique yielding one candidate event with an estimated background of 0:30 0:03 events. Combining this result with the observation of two candidate events by the predecessor experiment E787 gave the branching ratio

An event consistent with the signature expected for the rare kaon decay K 1 ! p 1 nn has been observed. In the pion momentum region examined, 211 , P , 230 MeV͞c, the backgrounds are estimated to contribute 0.08 6 0.03 events. If the event is due to K 1 ! p 1 nn, the branching ratio is 4.2 19.7 23.5 3 10 210 . [S0031-9007 (97)04229-4] PACS numbers: 13.20.Eb, 12.15.Hh, 14.80.MzThe decay K 1 ! p 1 nn has attracted interest due to its sensitivity to jV td j, the coupling of top to down quarks in the Cabibbo-Kobayashi-Maskawa quark mixing matrix. Theoretical uncertainty in the branching ratio is minimal because the decay rate depends on short distance physics and because the hadronic matrix element can be extracted from the well-measured decay K 1 ! p 0 e 1 n. After next-to-leading-logarithmic analysis of QCD effects [1], calculation of isospin breaking, phase space differences, and other small corrections to the hadronic matrix element [2], and calculation of two-electroweak-loop effects [3], the intrinsic uncertainty is only about 7% [4]. Based on current knowledge of standard model (SM) parameters, the branching ratio B͑K 1 ! p 1 nn͒ is expected to be in the range ͑0.6 1.5͒ 3 10 210 [5]. Longdistance contributions to the branching ratio (i.e., meson, photon exchange) appear to be negligible (10 213 ) [6,7]. Since K 1 ! p 1 nn is a flavor changing neutral current process that is highly suppressed in the SM, it also serves as a hunting ground for non-SM physics. The signature K 1 ! p 1 "nothing" [6,8,9] includes K 1 ! p 1 nn with non-SM intermediate states (such as virtual supersymmetric particles), K 1 ! p 1 nn 0 (a lepton flavor violating final state), K 1 ! p 1 X 0 X 0 0 where X 0 and X 0 0 are not neutrinos, and K 1 ! p 1 X 0 where X 0 is a single, noninteracting particle. Initial results from the E787 experiment [10] at the Alternating Gradient Synchrotron (AGS) of Brookhaven National Laboratory gave 90% confidence level (C.L.) upper limits B͑K 1 ! p 1 nn͒ , 2.4 3 10 29 and B͑K 1 ! p 1 X 0 ͒ , 5.2 3 10 210 for a massless X 0 [11]. In this Letter, we report on the analysis of a new data sample with 2.4 times greater sensitivity, taken in 1995 using an upgraded beam and detector.The signature for K 1 ! p 1 nn is a K 1 decay to a p 1 of momentum P , 227 MeV͞c and no other observable product. Definitive observation of this signal requires suppression of all backgrounds to well below the sensitivity for the signal and reliable estimates of the residual background levels. Major background sources include the copious two-body decays K 1 ! m 1 n m (K m2 ) with a 64% branching ratio and P 236 MeV͞c and K 1 ! p 1 p 0 (K p2 ) with a 21% branching ratio and P 205 MeV͞c. The only other important background sources are scattering of pions in the beam and K 1 charge exchange (CEX) reactions resulting in decays K 0 L ! p 1 l 2 n, where l e or m. To suppress the backgrounds, techniques were employed that incorporated redundant kinematic and particle identification measurements and efficient elimination of events with additional...

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We report the first measurement of a structure-dependent component in the decay K+-->&mgr;(+)nu(&mgr;)gamma. Using the kinematic region where the muon kinetic energy is greater than 137 MeV and the photon energy is greater than 90 MeV, we find that the absolute value of the sum of the vector and axial-vector form factors is |F(V)+F(A)| = 0.165+/-0.007+/-0.011. This corresponds to a branching ratio of B(SD+) = (1.33+/-0.12+/-0.18)x10(-5). We also set the limit -0. 04

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