6 Rare Kaon Decays B-counter C-counter D-counter

S. Pislak, P. Robmann and P. Tru ol, in collaboration with J. Egger, W. D. Herold, H. Kaspar, and H. Weyer, Paul{Scherrer{Institut, CH-5234 Villigen Brookhaven National Laboratory, Upton, NY-11973, USA University of New Mexico, Albuquerque, NM-87131, USA University of Pittsburgh, Pittsburgh, PA-15260, USA Yale University, Newhaven, CT-06511, USA Institute for Nuclear Research, Academy of Sciences, 117 312 Moscow, Russia Experiment E-865 at Brookhaven AGS The 1996 beam period turned out to be quite successful. Data were acquired about a week into the run and no major problems occured during the following 16 weeks. Compared to 1995 the e ciencies were improved for the large multiwire proportional chambers P3 and P4, a fourth trigger counter set (D, see Figure 6.1) was installed and the timing tightened at various points. This allowed a reduced trigger and a higher data taking rate giving a sensitivity to the branching ratio for K+ ! + +e of about 10 11. In parallel the analysis of the 1995 data with respect to this lepton avor conservation violating decay progressed. Preliminary results were presented at several conferences [1; 2; 3], and the thesis of Stefan Pislak [4] contains the nal result, which we discuss brie y below.