| A large number of astrophysical observations indicate that 27% of the universe's total energy comes from dark matter,which is impossible to be observed by optical method.In contemporary physics,there exist numerous theoretical models serving as microcosmic explanation of dark matter,among which two models have been most studied: WIMPs and Axions.The detection of dark matter is a frontier issue in today's particle physics research.Since the beginning of this century,there have been more than twenty experimental groups conducting dark matter direct detection experiments,trying to detect the signals caused by the collision between the WIMP dark matter particle and the ordinary matter's nucleus.Some experimental group even declared that they had found the signals of suspected dark matter,which caught the eyes of a wide range of particle physicists.Dark matter direct detection experiments verify its existence from observable signals caused by the elastic collision of WIMPs and the nuclei of target material(detection medium),of which the probability is extremely small.Therefore,the control of background is one of the biggest challenges in these experiments.In order to reduce the amount of background signals from energetic cosmic rays,dark matter direct detection experiments are usually carried out deep under the ground.In addition,this kind of experiment demands appropriate target material with low-level radioactivity to generate observable signals.At present,HPGe crystal and inert liquids such as argon and xenon constitute the best target materials in dark matter direct detection,and are used in experiments of highest sensitivity up to now.PandaX is a dark matter direct detection experiment employing liquid xenon as detection medium,and situated in China Jinping Underground Laboratory(CJPL).The first phase of PandaX project was set up in 2009,and the data taking started from May2014.On October 2014,the first results of data analysis had been published.To ensure the operation of experiment,PandaX contains a specially-designed cryogenic system.This dissertation consists of a detailed presentation of PandaX's cryogenic system(including its structural principle and its routine operation,maintenance and management),an introduction of the method of data analysis,and after all,a conclusion of the first results of data analysis of PandaX.Firstly,the dissertation is aimed at introducing PandaX's cryogenic system and its design.This cryogenic system ensures that PandaX detector could keep working steadily under a predetermined pressure and temperature.Besides,the system,with strong expansibility,can be applied by larger detector of several tons.The long-term and stable operation of the cryogenic system demands accurate daily manipulation,maintenance and management,including leak detection,pre-cooling,the filling of liquid xenon,the recirculation and purification of liquid xenon and the control of its liquid level.All these problems will also be discussed after this part.Then,the dissertation presents the procedure of PandaX's data analysis,including the calibration,reconstruction and correction of collected signals(so as to restore the signals of the nuclear recoils event in the detector).The text emphatically studies signal correction theory and procedure in terms of the detector's uniformity,and also its corresponding results.The first results of data analysis of PandaX will be demonstrated in the last part.These results are based on data collected with 37 kg liquid target mass(fiducial mass)in 17.4 livedays(valid period),and they turn out to be incompatible with the signals of suspected dark matter declared by some other laboratories. |
PDF Full Text Request