| Summary: | <p>The fundamental nature of dark matter stands as one of the greatest mysteries in physics. A compelling body of evidence indicates that dark matter accounts for 84% of the total matter density of the universe, and yet the detection of its constituent particles remains elusive. Stationed deep underground, the LUX-ZEPLIN (LZ) experiment utilises several tonnes of xenon in a dual-phase time projection chamber to conduct direct searches for particle dark matter. A world-leading limit was recently set by the LZ experiment for interactions between nucleons and weakly interacting massive particles, the most promising dark matter candidate. The work of this thesis leverages the unprecedented sensitivity and versatility of the LZ detector to expand the scientific programme to searches for rare phenomena beyond dark matter.</p>
<p>A comprehensive background model of detector materials is constructed in this work, using a combination of real data and simulations within a novel approach, as informed by expectations from an extensive radioassay campaign. This result is primarily applicable to searches for hypothesised neutrinoless double beta decays, the observation of which would shed light on the underlying properties of massive neutrinos, and is tied to the matter-antimatter asymmetry of the universe. With a large quantity of xenon, an ultra-low background environment, and excellent energy resolution, LZ is projected to achieve a sensitivity that is competitive with dedicated experiments.</p>
<p>The first LZ measurement of Xe-124 decay through the process of two-neutrino double electron capture is also presented here. This is the rarest nuclear decay observed to date, with a half-life that significantly exceeds the age of the universe. The dominant I-125 background is constrained with a detailed neutron activation model, and the exposure is maximised with an optimised fiducial volume. A half-life of (1.09 ± 0.18 stat ± 0.03 sys) × 10^22 years is observed with a statistical significance of 7σ. In future runs, the discovery of even rarer Xe-124 decay modes, namely two-neutrino electron capture with associated positron emission, will be within the physics reach of LZ.</p>
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