Search For Annual Modulation Signals From Dark Matter With The XENON100 Liquid Xenon Detector

Dark matter is one of the most critical problem in modern physics,and scientists all over the world are spearing no efforts to develop new techniques to decipher the nature of dark matter.Among these efforts,direct dark matter detections using dual phase xenon detectors are the most promising techniques which is probably able to detect dark matter signals in the coming years.Beside this,the DAMA/LIBRA experiments,which runs at the Gran Sasso national underground lab in Italy,claimed to have discovered the annual modulation signal consistent with that from the scatter of dark matter off atomic target.In the meanwhile,the XENON100 dual phase xenon detector,running at the same laboratory,accumulated 225 live days of dark matter search data from March,2011 to April 2012.The main topic of this thesis is to search for dark matter signals in the XENON100 data and make comparisons with the DAMA/LIBRA experiments.In Chapter 1,we introduce the dark matter problem and discuss about the prospective dark matter search experiment with a focus on the dual phase xenon technology.In Chapter 2,we mainly discuss the operation principles of dual phase xenon detector.We show how scintillation and ionisation signals are generated and detected in the detector,and focus our discussion on the fluctuations and characteristics of the signals.Based on that,we start analysing XENON100 data in Chapter 3.The main body of the chapter is to develop analysis cuts,to build the background model and to develop the profile likelihood analysis technique.In particular,we highlight the observation and application of single electron S2 signals which is from the photon-ionisation of impurities in the XENON100 detector.Base on this model,we develop a new method to identify and reduce neutron background by a factor of 60%for dark matter searches.Using the nuclear recoil and electronic recoil calibration data,we build the background model for use in the profile likelihood analysis to search for nuclear recoil dark matter signals in the XENON100data.In the 225 live days of XENON100 data,2 events are observed in the dark matter search region with a background expectation of 0.96_-0⁺⁰..₁₅²⁰.The profile likelihood analysis confirms that there is no significant signal excess from dark matter over the full mass range the experiment is sensitive to.We thus use the profile likelihood analysis to produce the exclusion limit on dark matter nucleon cross sections over the full mass range.This results excluded much of the param-eter space favours by popular particle physics models and the interpretation of DAMA/LIBRA modulations as being from nuclear recoil signals of dark matter.Since the interpretation of DAMA/LIBRA modulation as nuclear recoil signals has been excluded,some theorists proposed several particle physics models in which dark matter only produce electronic recoils in the detector,and hence the experimental constraints from nuclear recoil searches are avoided.The XENON100 electronic recoil data,which is accumulated by more than one year,offers a great chance to search for electronic recoil annual modulation signals from dark matter and to test the DAMA/LIBRA results in this framework.In Chapter 4,we firstly analyse the variations of the parameters that characterises the stabil-ity of the XENON100 detector and perform a correlation test with the event rate.We conclude from this analysis that the stability of XENON100 detector is sufficient for search of modulation signals.Due to a tiny air leak in the circulation system,the concentration of radon and krypton atoms in xenon inhabit variations in time and leads to time dependent contribution to the elec-tronic recoil background.Based on this,we develop the full background model and applied it in the profile likelihood analysis to search for annual modulation signals in the XENON100 dark matter search data.In the low energy region of?2-6?keV,annual modulation signatures are observed for both single scatter and multiple scatter data,but its phase is 2.6?away from the standard halo model.No modulation signature is found in the high energy region of?6-10?keV.Assuming that the annual modulation signals are from dark matter scatters off electrons via axial-vector interaction,the DAMA/LIBRA modulation is excluded by XENON100 data at 4.8?.