Fast pulse shape analysis for rare kaon decay experiments

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The rare kaon decay KAPPA<@D1+@>D1 *pi<@D1+@>D1nu<@D4nu@>D4, a flavor changing neutral current process, is strongly suppressed by the Glashow-Iliopoulos-Miani Mechanism. The branching ratio, which mainly depends on the Kobayashi-Maskawa matrix element |Vtd|, is predicted to be (1.1(]SY.+-。[)0.4)*10<@D1-10@>D1 by the Standard Model, while the present upper limit is 2.4*10<@D1-9@>D1 (90% c.l.). In order to improve this experimental sensitivity, we upgraded our detector system and developed various methods for background rejection, namely rejection of pi<@D1+@>D1pi<@D10@>D1, mu<@D1+@>D1nu, mu<@D1+@>D1nu gamma decay events.This project aimed to develop a fast pulse separation method, by which occurrence of background event just before or just after triggering can be reliably identified even if two pulses are piled up. Analog output pulses from each module in the E787 gamma veto detector were recorded with a high speed transient digitizer and analyzed through on-line and off-line program codes. Since precise time and energy information is indispensable to obtain a good veto efficiency for gamma-ray and a low accidental veto rate, we carefully investigated pulse shapes of outputs from all channels of the gamma veto detector and tuned the analysis code.In the 1995 physics run at BNL,we took data written on 4200 tapes and studied detector response to the background process, KAPPA^+ *pi^+pi^0, as well as data analysis for KAPPA^+ *pi^+nu^^<nu>. We found that a time resolution of better than 1ns was obtained for low-energy photons down to 10 MeV.The accidental event rate at 1.2*10^6 (kaons stopping on our target)/pulse was about 5%. We obtained a rejection factor of 1*10^6 to KAPPA^+ *pi^+pi^0 decay (with 25% acceptance loss) through the analysis code which we developed in this project