Selected Topics On Atomic Processes In Direct Dark Matter Detection Experiments

Exploring the dark matter problem is one of the most important research topics in elementary particle physics,astrophysics,astronomy and cosmology.Currently,accumulating evidences in astronomy have indicated that there is a large amount of non-luminous dark matter in the universe.Therefore,direct detection of dark matter particle becomes an extremely significant and urgent issue.Most of the dark matter direct detection experiments search for dark mat-ter particles by collecting the ionization,scintillation and heat signals produced through dark matter particles scattering with the detector atoms.In the detect-ing experiments,detectors are in the atomic environment,and there are various kinds of atomic processes.Thus,the understanding of atomic processes in de-tectors turns out to be decisive in the direct detection experiments.Atomic processes play a crucial role in the dark matter direct detection not merely the background understanding but opening the detection channels for unknown dark matter particles.Therefore,it is necessary to study the effects of these atomic processes on the dark matter direct detection.Based on the requirements of dark matter direct detection experiments,two typical atomic processes are studied in this dissertation.They are atomic Compton scattering process and atomic ion-ization process induced by millicharged particles.Atomic Compton scattering is one of the dominant X-ray and gamma-ray background processes in dark matter direct detection experiments,and the study of atomic Compton scattering shall contribute to the background analysis of these experiments.Millicharged parti-cles are a type of subatomic particles with a tiny fraction of the electric charge proposed by theories beyond the Standard Model.The atomic ionization pro-cess induced by millicharged particles is the detection channel for millicharged particles.The study of ionization process induced by millicharged particles shal-1 help us experimentally search for millicharged particles and further constrain their physical parameters.In the study of atomic Compton scattering,the relativistic impulse approx-imation(RIA)approach is adopted to the Compton scatterings of Si,Ge,Ar,Xe atoms.These elements form the materials of detectors in dark matter direct detection experiments.The scattering function of atomic Compton scattering is calculated and analyzed,and the differential cross section as well as the energy spectrum in Compton scattering processes is studied.To consider the relativistic effect,we adopt the fully relativistic Dirac-Fock and Multi-Configuration Dirac-Fock theories in the atomic calculations to obtain the atomic ground state wave-functions.Our results indicate that the atomic Compton scattering can be greatly influenced by atomic many-body effects in low energy transfer or the low momen-tum transfer regions.Our theoretical results could be confirmed by experiments in the future.Furthermore,in the study of atomic Compton scattering,an improved treat-ment of RIA is developed,and the calculational results are compared with those from former treatments of RIA.This new treatment of RIA can numerically test the validity of the simplified approximations introduced by Roland Ribberf'ors et al.in former RIA treatments.The comparison results show that these simplified approximations in former treatments of RIA work well when final photon energy is near the Compton peak region.However,when final photon energy goes far away from the Compton peak,some approximations introduced by Roland Rib-berfors et al.break down.Through the comparisons with S-Matrix results and experimental measurements,it is observed that our new treatment of RIA is still imprecise in region away from the Compton peak.This is due to the limitations in RIA approach:the many-body effects are only reflected in the electron kine-matics,and they are not efficiently treated in the dynamical process.Further study with more advanced approaches are leaving for future.In the study of the millicharged particles,the RIA approach is successfully applied to the atomic ionization process induced by millicharged particles.The formulation of RIA is derived for atomic ionization induced by millicharged par-ticles,and the numerical calculations are obtained and compared with those from free electron approximation and equivalent photon approximation.Concretely,we study the ionization of Ge and Xe atoms induced by millicharged particles,calculating the differential cross section and estimating the event rate of these processes in real detectors.The estimation of reaction count rate in detectors indicates that,in the next-generation experiments with 100 eV thresholds and 0.1 count/kg·keV·day background levels,the detection sensitivity of millicharge would be ???10-8 for dark matter particles and ?v?10-12 for neutrinos.