A statistical framework for the characterisation of WIMP dark matter with the LUX-ZEPLIN experiment

Abstract

Several pieces of astrophysical evidence, from galactic to cosmological scales, indicate that most of the mass in the universe is composed of an invisible and essentially collisionless substance known as dark matter. A leading particle candidate that could provide the role of dark matter is the Weakly Interacting Massive Particle (WIMP), which can be searched for directly on Earth via its scattering off atomic nuclei. The LUX-ZEPLIN (LZ) experiment, currently under construction, employs a multi-tonne dual-phase xenon time projection chamber to search for WIMPs in the low background environment of the Davis Campus at the Sanford Underground Research Facility (South Dakota, USA). LZ will probe WIMP interactions with unprecedented sensitivity, starting to explore regions of the WIMP parameter space where new backgrounds are expected to arise from the elastic scattering of neutrinos off xenon nuclei.

In this work the theoretical and computational framework underlying the calculation of the sensitivity of the LZ experiment to WIMP-nucleus scattering interactions is presented. After its planned 1000 live days of exposure, LZ will be able to achieve a $3\sigma$ discovery for spin independent cross sections above 3.0e-48 cm^2 at 40 GeV/c^2 WIMP mass or exclude at 90\%~CL a cross section of 1.3e-48 cm^2 in the absence of signal. The sensitivity of LZ to spin-dependent WIMP-neutron and WIMP-proton interactions is also presented. All the sensitivity projections are calculated using the LZStats software package, which is discussed in detail in this thesis. In addition, this work classifies key systematic uncertainties by their impact on the WIMP sensitivity and motivates the inclusion of the highest-ranked into the analysis likelihood function. The effect of some of these systematics on the reconstruction of the WIMP cross section is also studied and it is found to be sub-dominant.