Improving Energy Reconstruction and Background Rejection of EXO-200

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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.