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...

No abstract

No abstract

No abstract

The results of total cross section measurements are presented for~' on targets of natural Li, C, Al, Fe, Sn, and Pb in the region of 65 -320 MeV laboratory kinetic energy.The data are fitted with a simple phenomenological model, which allows one to extract the A dependence of the peak energy and width which characterize the pion-nucleus interactiorr.There have been several recent experiments to measure pion-nucleus total cross sections on light nuclei in the energy region corresponding to the (3, 3) resonance in the elementary vN interaction. 'Ĩ n the present note, we present results for total cross sections o", of v' on a range of nuclei from Li to Pb. One of the principal motivations of the experiment was to study the dependence of the position and width of the peak in 0", on target mass number A. The shift and broadening of the peak has been discussed theoretically in terms of multiple scattering effects, ' the Pauli principle, ' and "collisions damping" processes. ' The latter two effects lead to modifications of the pion-nucleon amplitude in the nucleus. Calculations with simple first order optical potentials8 [proportional to the nuclear density p(r) times the Fermi-averaged free space vN amplitude] do not correctly predict the position or the width of the peak in cr",. Since the present experiment provides the first measurements involving heavy targets, the results may be useful in untangling the various competing mechanisms which shift and broaden the mN isobar in the nucleus.We have performed measurements of the total cross sections for 7t' at 14 lab kinetic energies between 65 and 320 MeV, and for m at 8 energies between 80 and 320 MeV, on natural targets of Li, C, Al, Fe, Sn, and Pb. We used the same low energy separated beam from the AGS and the same detector system as in two previous experiments. '" Incident pions were selected by a differential Cerenkov counter and proportional wire chamber system. The nuclear targets were 15 cm && 15 cm && 5 g/cm' thick, and were positioned just downstream of the liquid-hydrogen-deuterium target system used in the previous experiments. The simultaneous measurements on hydrogen and deuterium will be "eported elsewhere. At each inci-dent energy, the transmission of the beam was measured, for no target and for each of the six nuclear targets, with nine circular scintillation counters of different size. Compensation for particle absorption was made electronically. " The counters were always positioned corresponding to a fixed region of four-momentum transfer~tf rom 0.002 to 0.004 (GeV/c)'. The total cross sections were obtained in the usual way by using a linear extrapolation to t; =0 of the partial cross sections o'", measured with each counter.Before extrapolating, it was important to make corrections to the observed 0'" to remove the effects of (1) beam decay losses a, "' which were different for target and no-target measurements, and (2) Coulomb a~c" and Coulomb-nuclear interference chic'"' corrections. The values o~"' were computed by a Monte Carlo method, which i...