PandaX-? Experiment For Low-mass Dark Matter Search

In 2015,the Science Journal listed 125 unanswered questions that face the scientific community over the next quarter-century.And among these big questions,the top one is What's the Universe made of ? Observations from the modern cosmology indicate that there is a dominating amount of ”unseen” matter in the universe,which is the so-called Dark Matter.To explain such missing matter,theoretical physicists have proposed many candidate particles.However,up to date,we have not convincingly detected any dark matter particles and we still know nothing about the particle porperity of the dark matter.To date,searching for dark matter is still the most pressing,but also the most challenging fundamental physics question.Finding the dark matter in the laboratory and elucidating their properties will most likely revolutionalize our understanding on cosmic evolution,fundamental building blocks of the matter,as well as interactions between the fundamental particles.Among the proposed dark matter candidates,the Weakly Interaction Massive Particles or WIMPs are the most theoretically motivated ones.Around the globe,many experiments are ongoing to detect such dark matter particles.The PandaX(Particle AND Astrophysics experiment with Xenon)Experiment is a staged experiment using xenon as the target operating at the China Jin-Ping Underground Laboratory in the Sichuan Province.The first stage PandaX-I Experiment,the main focus of this thesis,is the first dark matter experiment using the advanced dual-phase liquid xenon time ptojection chamber technology in China.In this thesis,the details of the working principle of the PandaX-I Experiment,the installation and operation of the detector,as well as the final data analysis will be presented.As an abstract,here we give a brief introduction to each chapter:Chapter.1 of this thesis is the introduction to the dark matter.We introduce the cosmological origin of the dark matter,the candidates of dark matter particles proposed by theoretical physicists,and the dark matter detection.In this chapter,we survey the worldwide experiments searching for WIMP dark matter,which include both the direct and indirect detection,as well as collider search.In Chapter.2,we focus on the discussion of dual-phase liquid xenon time projection chamber technology for dark matter direct detection.We discuss the properties of the xenon as detector medium,the working principle of dual-phase liquid xenon TPC,and the background discrimination power for dark matter experiments.Then in the next two chapters,we present details of the hardware and software of the PandaX-I experiment.It covers main components of the detector including the passive shielding,cryogenics system,the TPC and PMT system,the electronics and data acquisition system,the feedthroughs and cabling of the detector,the calibration system,the leveling adjustment and the distillation tower for radon removal.The overall running status of the detector will be presented in this chapter too.In Chapter.4,we will introduce the data transfer,storage and data quality control,reduction,and the main data quality cuts.Chapter.5 and 6 are the main contents of this thesis.Here we will present details of detector's responses including trigger performance,position non-uniformity,and different charge of response to electricial fields.We also make full discussion on the parameters of the detector such as the light collection efficiency,electron extraction efficiency,energy resolution and so on.We then focus on the detector calibration including the characterization of the photomultiplier tubes,and the nuclear recoil and electron recoil calibration.Based on the calibration,we predict the background in the dark matter running.To take a blind analysis for PandaX-I full exposure data,we determine key selection cuts before examining the data in the dark matter search region.The detailed procedure on setting the cuts is given in Chapter.7.Finally,we present the dark matter search results of the PandaX-I experiment in Chapter.8.We introduce the fitting method with which we interpret the experimental data to the limits on dark matter-nucleon scattering cross sections.We conclude by highlighting the achievements in PandaX-I as well as presenting an outlook to the second stage PandaX-II experiment.