Fast pulse shape analysis for rare kaon decay experiments

罕见的kaon衰变KAPPA<@D1+@>D1 *pi<@D1+@>D1nu<@ d4nu @>D4是一种变味中性电流过程，被glashowi - iliopoulos - miani机制强烈抑制。分支比主要依赖于Kobayashi-Maskawa矩阵元素|Vtd|，标准模型预测分支比为(1.1(]SY.+- [)0.4)*10<@D1-10@>D1，而目前的上限为2.4*10<@D1-9@>D1 （90% c.l）。为了提高实验灵敏度，我们对探测器系统进行了升级，并开发了多种本底抑制方法，即抑制pi<@D1+@>D1pi<@ d10 @>D1, mu<@D1+@>D1nu, mu<@D1+@>D1nu伽马衰变事件。本项目旨在开发一种快速脉冲分离方法，即使两个脉冲叠加，也能可靠地识别出触发前或触发后背景事件的发生。用高速瞬态数字化仪记录E787伽马否决权探测器各模块的模拟输出脉冲，并通过在线和离线程序代码进行分析。由于精确的时间和能量信息是获得良好的伽马射线否决效率和低意外否决率所不可或缺的，我们仔细研究了伽马否决探测器所有通道输出的脉冲形状，并对分析代码进行了调整。在1995年BNL的物理运行中，我们将数据写在4200个磁带上，并研究了探测器对背景过程KAPPA^+ *pi^+pi^0的响应，以及KAPPA^+ *pi^+nu^^<nu>的数据分析。我们发现，对于低至10 MeV的低能光子，获得了优于1ns的时间分辨率。在1.2*10^6（介子停在目标上）/脉冲时，意外事件率约为5%。通过我们在这个项目中开发的分析代码，我们获得了1*10^6到KAPPA^+ *pi^+pi^0衰减的拒绝因子（接受损失为25%）

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