The detector of the E865-collaboration at the Brookhaven-AGS described here combines a magnetic spectrometer for the charged decay products of 6GeV/cK+ with excellent electromagnetic calorimetry and efficient particle identification for electrons and muons. Its high-resolution, large acceptance and high rate capability made it well suited for the study of extremely rare or forbidden decays with multi-leptonic final states such as K+→π+μ+e−, K+→π+l+l−, K+→l+νle−e+ and K+→π+π−e+νe down to branching ratios below 10−11 in an intense K+ beam (≈108 per AGS spill).

R. Appel,6,3 G. S. Atoyan,4 B. Bassalleck,2 D. R. Bergman,6,* D. N. Brown,3,† N. Cheung,3 S. Dhawan,6 H. Do,6 J. Egger,5 S. Eilerts,2,‡ C. Felder,3,1 H. Fischer,2,§ M. Gach,3,k W. Herold,5 V. V. Issakov,4 H. Kaspar,5 D. E. Kraus,3 D. M. Lazarus,1 L. Leipuner,1 P. Lichard,3 J. Lowe,2 J. Lozano,6, H. Ma,1 W. Majid,6,** W. Menzel,7, S. Pislak,8,6 A. A. Poblaguev,4 V. E. Postoev,4 A. L. Proskurjakov,4 P. Rehak,1 P. Robmann,8 A. Sher,3 J. A. Thompson,3 P. Truöl,8,6 H. Weyer,7,5 and M. E. Zeller6 1Brookhaven National Laboratory, Upton L. I., New York 11973 2Department of Physics and Astronomy, University of New Mexico, Albuquerque, New Mexico 87131 3Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 4Institute for Nuclear Research of Russian Academy of Sciences, Moscow 117 312, Russia 5Paul Scherrer Institut, CH-5232 Villigen, Switzerland 6Physics Department, Yale University, New Haven, Connecticut 06511 7Physikalisches Institut, Universität Basel, CH-4046 Basel, Switzerland 8Physik-Institut, Universität Zürich, CH-8057 Zürich, Switzerland (Received 27 April 2000) We report results of a search for the lepton-family number violating decay K ! pme from data collected by experiment E865 in 1996 at the Alternating Gradient Synchroton of Brookhaven National Laboratory. We place an upper limit on the branching ratio at 3.9 3 10211 (90% C.L.). Together with results based on data collected in 1995 and an earlier experiment, E777, this result establishes a combined 90% confidence level upper limit on the branching ratio at 2.8 3 10211. We also report a new upper limit on the branching ratio for p0 ! me of 3.8 3 10210 (90% C.L.).

We report results of a search for the lepton-family number violating decay K+-->pi(+)&mgr;(+)e(-) from data collected by experiment E865 in 1996 at the Alternating Gradient Synchroton of Brookhaven National Laboratory. We place an upper limit on the branching ratio at 3.9x10(-11) ( 90% C.L.). Together with results based on data collected in 1995 and an earlier experiment, E777, this result establishes a combined 90% confidence level upper limit on the branching ratio at 2.8x10(-11). We also report a new upper limit on the branching ratio for pi(0)-->&mgr;(+)e(-) of 3.8x10(-10) ( 90% C. L.).

We report results of a search for the lepton-family number violating decay K+-->pi(+)m(+)e(-) from data collected by experiment E865 in 1996 at the Alternating Gradient Synchroton of Brookhaven National Laboratory. We place an upper limit on the branching ratio at 3.9x10(-11) ( 90% C.L.). Together with results based on data collected in 1995 and an earlier experiment, E777, this result establishes a combined 90% confidence level upper limit on the branching ratio at 2.8x10(-11). We also report a new upper limit on the branching ratio for pi(0)-->m(+)e(-) of 3.8x10(-10) ( 90% C. L.).

Experiment E865 at the BNL AGS is a search for the Lepton Family Violating decay K+→π+μ+e−1. We aim to reach a single event sensitivity below 10−11 for this decay. The E865 apparatus allow us to collect and study other rare and semi rare K+ decays. Preliminary results are shown on K+→π+μ+e−, K+→π+e+e−, K+→π+μ+μ−, K+→π+π−e+ν, K+→e+νe+e−, K+→μ+νe+e−, K+→π0e+ν (π0→e+e−γ).

1Brookhaven National Laboratory, Upton Long Island, New York 11973 2Department of Physics and Astronomy, University of New Mexico, Albuquerque, New Mexico 87131 3Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 4Institute for Nuclear Research of Russian Academy of Sciences, Moscow 117 312, Russia 5Paul Scherrer Institut, CH-5232 Villigen, Switzerland 6Physics Department, Yale University, New Haven, Connecticut 06511 7Physikalisches Institut, Universität Basel, CH-4046 Basel, Switzerland 8Physik-Institut, Universität Zürich, CH-8057 Zürich, Switzerland (Received 21 July 1999 )

A large low-background sample of events (10300) has been collected for the rare decay of kaons in flight K+ -> pi+ e+ e- by experiment E865 at the Brookhaven AGS. The decay products were accepted by a broad band high-resolution charged particle spectrometer with particle identification. The branching ratio (2.94 +- 0.05(stat.) +- 0.13(syst.) +- 0.05(model))*10**{-7} was determined normalizing to events from the decay chain K+ -> pi+ pi0; pi0 -> e+ e- gamma. From the analysis of the decay distributions the vector nature of this decay is firmly established now, and limits on scalar and tensor contributions are deduced. From the (e+ e-) invariant mass distribution the decay form factor f(z)=f0(1+ delta*z) (z=M(ee)**2/m(K)**2) is determined with delta=2.14 +- 0.13 +- 0.15. Chiral QCD perturbation theory predictions for the form factor are also tested, and terms beyond leading order O(p**4) are found to be important.

A low-background sample of 10 300 events has been collected for the decay mode ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{e}^{+}{e}^{\ensuremath{-}}$ by experiment E865 at the Brookhaven Alternating Gradient Synchrotron. The branching ratio is measured to be $[2.94\ifmmode\pm\else\textpm\fi{}0.05(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.13(\mathrm{syst})\ifmmode\pm\else\textpm\fi{}0.05(\mathrm{model})]\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$, the vector nature of this decay is firmly established, and the decay form factor $f(z){\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}f}_{0}(1+\ensuremath{\delta}z)({z\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}M}_{\mathrm{ee}}^{2}{/m}_{K}^{2})$ is determined to have $\ensuremath{\delta}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2.14\ifmmode\pm\else\textpm\fi{}0.13\ifmmode\pm\else\textpm\fi{}0.15$. The experiment is described and results discussed in the context of chiral perturbation theory.

We propose a new experiment at the AGS to search for the T-violating polarization of the muon normal to the decay plane of the K+ to mu+ pi0 nu decay. Motivated by the need for a stronger CP violation source to account for the baryon asymmetry of the Universe, the experiment aims to search for T-violation beyond the Standard Model. The experiment will be performed with in-flight decays from an intense 2 GeV/c separated K+ beam at the AGS. We expect to analyze 10**9 events to obtain the sensitivity of delta Pt = +- 0.00013 at 1 sigma, corresponding to the sensitivity of +-0.0007 to Im(xi), an improvement by 40 over the present limit.

Results of a search for the decay mode K(L)0 --> gamma-gamma-ee (K(L)0 --> gamma-ee with a hard internal bremsstrahlung) are reported. The measured branching ratio is (6.6 +/- 3.2) x 10(-7), in agreement with the QED prediction of 5.8 x 10(-7). The infrared cutoff for both branching ratios is the requirement that both photons have an energy of at least 5 MeV on the center of mass.