KL to pi0 e+ e- and KL to pi0 mu+ mu- : A binary star on the stage of flavor physics

A systematic analysis of New Physics impacts on the rare decays KL to pi0 l+ l- is performed. Thanks to their different sensitivities to flavor-changing local effective interactions, these two modes could provide valuable information on the nature of the possible New Physics at play. In particular, a combined measurement of both modes could disentangle scalar/pseudoscalar from vector or axial-vector contributions. For the latter, model-independent bounds are derived. Finally, the KL to pi0 mu+ mu- forward-backward CP-asymmetry is considered, and shown to give interesting complementary information.Comment: 20 pages, 5 figures. Comments and references added, minor numerical changes, conclusion unchanged, to appear in JHE

Improved measurement of the K+->pi+nu(nu)over-bar branching ratio

An additional event near the upper kinematic limit for K+-->pi(+)nu(nu) over bar has been observed by experiment E949 at Brookhaven National Laboratory. Combining previously reported and new data, the branching ratio is B(K+-->pi(+)nu(nu) over bar)=(1.47(-0.89)(+1.30))x10(-10) based on three events observed in the pion momentum region 211<P<229 MeV/c. At the measured central value of the branching ratio, the additional event had a signal-to-background ratio of 0.9

Study of the decay K+ => pi+,nu,nubar in the momentum region 140<Ppi<199 MeV/c

Experiment E949 at Brookhaven National Laboratory has observed three new events consistent with the decay K+ => pi+,nu,nubar in the pion momentum region 140 < P_pi < 199 MeV/c in an exposure of 1.71e12 stopped kaons with an estimated total background of 0.93+-0.17(stat.)+0.32-0.24(syst.) events. This brings the total number of observed K+ => pi+,nu,nubar events to seven. Combining this observation with previous results, assuming the pion spectrum predicted by the standard model, results in a branching ratio of (1.73+1.15-1.05)e-10. An interpretation of the results for alternative models of the decay K^ => pi+,nothing is also presented.Comment: Submitted to Phys.Rev.

New Measurement of the K+->pi(+)nu nu Branching Ratio

Three events for the decay K+->pi(+)nu have been observed in the pion momentum region below the K+->pi(+)pi(0) peak, 140 pi(+)nu nu)=(1.73(-1.05)(+1.15))x10(-10) consistent with the standard model prediction

Evidence for K{sup +} {r_arrow} {pi}{sup +} {nu}{anti {nu}}

The decay K{sup +} {r_arrow} {pi}{sup +}{nu}{anti {nu}} has been observed for the first time. The E787 experiment has presented evidence for the K{sup +} {r_arrow} {pi}{sup +}{nu}{anti {nu}} decay, based on the observation of a single clean event from data collected during the 1995 run of the AGS (Alternating Gradient Synchrotron at Brookhaven National Laboratory). The branching ratio indicated by this observation, B(K{sup +} {r_arrow} {pi}{sup +}{nu}{anti {nu}}) = 4.2{sub {minus}3.5}{sup +9.7} {times} 10{sup {minus}10}, is consistent with the Standard Model expectation although the central experimental value is four times larger. The final E787 data sample, from the 1995--99 runs, should reach a sensitivity of about eight times that of the 1995 run alone. A new experiment, E949, has been approved to run, starting in the year 2000, and is expected to achieve a sensitivity of more than an order of magnitude below the prediction of the Standard Model