Background Discrimination For The Neutrinoless Double Beta Decay In The Future Large Liquid Xenon Experiment With Cherenkov Light

As the research on neutrino oscillation gets deeper and deeper,more neutrino physical problems emerge,one of which is to determine whether the neutrino is Dirac particle or Majorana particle,it is significant for the origin of neutrino mass,experimentally through neutrinoless double beta decay(NLDBD).Many in-ternational experiments,such as EXO,NEXT and KamLAND-Zen,are searching for the NLDBD of 136Xe.As the dark matter detectors enter into the ton scale,NLDBD research can also be carried out at the same time.The PandaX series of experiments located in China Jinping Underground Laboratory(CJPL)is also one of them.The PandaX experimental group is composed of members from more than ten research institutions such as Shanghai Jiaotong University and Shan-dong University.The PandaX-4T detector under commissioning operation and future PandaX-30T detector have the ability to search for NLDBD in addition to dark matter particles.The XENONnT experiment at the Gran Sasso National Laboratory in Italy,the LZ experiment at the SURF laboratory in South Dakota,the United States,and the future DARWIN experiment all use two-phase TPC technology.While studying dark matter,they are also searching for NLDBD.In this thesis,I mainly introduce the physical target of NLDBD,and the back-ground discrimination of NLDBD experiment using 136Xe as the decay material.For the high-energy gamma rays with energy close to Q??,undergo photoelectric absorption without compton scattering,will produce a single high-energy electron with almost all the gamma's energy.This electron will produce a single inter-action site similar to the NLDBD decay,which cannot be eliminated by the site multiplicity cut.The usage of position distribution of Cerenkov light for this background discrimination will be introduced in this thesis.For future large-scale liquid xenon experiments,Geant4 is used to construct a simple liquid xenon detector.The generated optical signals include scintillation light and Cerenkov light.The differences in wavelength of these two optical signals result in differ-ent velocity in xenon and the Cerenkov light can be measured by the detected time with a certain efficiency.The NLDBD signal generates two electrons,and only one electron is generated by the high-energy gamma background undergo the photoelectric effect.Using the directivity of Cherenkov light,the position distribution of Cherenkov light generated by the dual-electron signal and the single-electron background case will be different.Under the premise of analyzing the single and double electrons emitted in a specific direction,the Cherenkov light distribution generated by electrons emitted in isotropic directions which is closer to the real situation is studied.At the end,use convolutional neural network to construct a simple model,perform image recognition on Cerenkov light dis-tribution map,and give preliminary signal selection efficiency and background rejection efficiency.The research in this thesis can provide a reference for the design of future large liquid xenon detectors.