UC Davis

Several independent studies have provided evidence supporting a cold, non-baryonic, non-luminous component of the universe, which is colloquially referred to as dark matter. Weakly interacting massive particles (WIMPs) are a well-motivated candidate to describe the particle nature of this component of the universe. There are several approaches to searching for evidence of these particles. This dissertation focuses on LUX-ZEPLIN (LZ), a direct dark matter search using a dual-phase xenon time projection chamber (TPC). Standard searches for WIMP recoils off xenon nuclei assume a spin-independent or spin-dependent interaction in the zero-momentum limit. This work focuses on a more general interaction basis derived from a non-relativistic effective field theory (EFT) approach to WIMP-nucleus interactions. More specifically, we set limits on the coupling strengths for the fourteen operators that form the basis of inelastic WIMP-nucleus interactions in the non-relativistic regime. The limit-setting analysis was conducted using an extended, unbinned profile likelihood ratio (PLR) method. World-leading limits are achieved for the lower mass-splitting values considered in the analysis. The second half of this dissertation expands on a key requirement in such dark matter searches: discrimination between nuclear (NR) and electronic recoil (ER) events. One of many methods to achieve this capability is pulse shape discrimination (PSD). Although xenon is not a suitable detection medium to take advantage of this technique, plastic/organic scintillators exhibit this capability. This subclass of scintillators are widely used in the radiation detection and nuclear security fields. This dissertation presents the design of and results from a custom SiPM + Scintillator test bed. The setup was used to study the PSD capability of commercially available plastic/organic scintillators when optically coupled to silicon photomultipliers (SiPMs). Also discussed are the results of a feasibility study, conducted using data taken with the test bed, on the design of a prototype fully custom application specific integrated circuit (ASIC) developed to read out signals from plastic/organic scintillators using SiPMs and provide real-time PSD via a custom analog circuit design on the ASIC. The end goal for this ASIC is integration into a compact, portable, and segmented neutron scatter camera design. The design, simulation, and initial testing results for the chip are presented. Condensed versions of the data sheets are provided as appendices for both versions of the chip that have currently been fabricated.