Improving Energy Reconstruction and Background Rejection of EXO-200

无中微子双β衰变可能提供了一个独特的机会来回答粒子物理学中的一个基本问题，即中微子是狄拉克或马约纳拉费米子的性质。这种衰变还没有被观察到。EXO-200是一个寻找Xe-136的无中微子双β衰变的实验。探测器是一个圆柱形的、位置分辨的时间投影室，里面装满了浓缩的液体氙气。该实验位于美国新墨西哥州的废物隔离中试工厂。无中微子双β衰变将通过测量闪烁光和液体氙气中衰变电子释放的电子数来探测。闪烁光将在雪崩光电二极管的两个平面上被探测到。二次电子在漂移过程中将通过收集(U线)和感应(V线)两种方式在两个线栅中被探测到。在第一个测量阶段，EXO-200已经与Gerda和Kamland Zen一起实现了对有效Majorana中微子质量的最佳限制之一。对于即将到来的从2016年开始的第二阶段数据采集，信号处理电子设备中的噪声被降低，从而触发阈值将被降低，能量分辨率将被提高。在这个拟议的项目中，将提高能量校准和能量重建的系统精度，从而提高对衰变半衰期的灵敏度。为实现这一目标，将对探测器的响应进行蒙特卡罗模拟，并改进模拟中的探测器建模。特别是对末态两个带电粒子的信号事件特征的电荷-光反相关模型进行了研究。除此之外，我们还将研究如何使用V线信号来提高能量分辨率。该项目还将改进对主要由探测器中多次光子散射引起的背景事件的识别。为此，应模拟U线和V线中的脉冲形式，并将其用于训练随机决策森林。通过随机决策森林的应用，提高了在一个位置有能量沉积的事件(单一地点事件，信号的特征)与在多个地点有能量沉积的事件(多地点事件，背景的特征)之间的分离强度。应开发一种新的分析方法，该方法基于测量检测到的事件大小的连续度量。

Neutrinoless double beta decay may offer a unique opportunity to answer a fundamental questions in particle physics, namely the nature of neutrinos being either Dirac or Majonara fermions. This decay has not yet been observed. EXO-200 is an experiment searching for neutrinoless double beta decay of Xe-136. The detector is a cylindrical, position resolving time projection chamber filled with enriched liquid xenon. The experiment is located in the Waster Isolation Pilot Plant in New Mexico,USA. The neutrinoless double-beta decay would be detected by the measurement of the scintillation light and the number of electrons released by the decay electrons in the liquid xenon. The scintillation light would be detected in two planes of avalanche photodiodes. The secondary electrons will be detected during their drift with two wire-grids by collection (U wires) and induction (V wires). During its first measuring phase, EXO-200 already has achieved, together with GERDA and KamLAND Zen, one of the best limits on the effective Majorana neutrino mass. For the upcoming phase 2 of data taking starting in 2016the noise in the signal processing electronics was reduced so that the trigger threshold will be lower and the energy resolution will be improved. During this proposed project, the energy calibration and the systematic accuracy of the energy reconstruction shall be improved resulting in a higher sensitivity on the half life of the decay.To achieve this goal, Monte-Carlo simulations of the detector response will be performed and the modeling of the detector in the simulation will be improved. Especially the modeling of the charge-light anticorrelation for the signal event signature with two charged particles in the final state will be investigated. In addition to that we will investigate how the V-wire signals can be used to improve the energy resolution. This project also will improve the identification of background events that are mostly caused by photons scattering several times in the detector. To do this, the pulse forms in the U and V wires shall be simulated and be used to train random decision forests. The separation strength between events with energy deposition at one position (single site event, signature of the signal) against events with energy depositions at several sites (multi site event, signature of background) shall be improved by the application of random decision forests. A new analysis method shall be developed which is based on a continuous metric measuring the detected size of an event.