Perry, Emily; (2024) Background Mitigation in LZ and Next Generation Rare Event Searches. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Dark matter has been indicated to make up 85% of the matter in the universe, and its discovery would deliver the first evidence of physics beyond the Standard Model (SM); bringing forward a new era in physics and astronomy. Weakly Interacting Massive Particles (WIMPs) arising naturally from SM extensions have been the focus of direct detection efforts, and dual-phase xenon (Xe) time projection chambers (TPCs) have been dominating this search for the past decade. The most sensitive of all Xe TPCs is LUX-ZEPLIN (LZ). From its first science run (SR1) of only 60 live-days the experiment has set a world-leading limit on the existence of WIMPs down to 9 GeV/c2. LZ will continue to explore uncharted electroweak parameter space, whilst also making the first discovery of coherent elastic neutrino nucleus scattering (CEνNS) from an astro-particle source, and deliver the first competitive neutrinoless double beta decay (0νββ) result from a direct dark matter detector. For many detector types including TPCs, radon is the dominant background due to the sub-chain “naked” beta emission from its progeny 214Pb. This decay was predicted to account for ∼ 66% of the LZ’s projected electron-recoil (ER) background. The work presented shows the development of novel methods to track and visualise the movement of 222Rn progeny through the detector. Informing a custom-built simulation of the 222Rn decay chain, used to predict the distribution of 214Pb, and in turn its activity inside the active xenon region of the detector. A major milestone result used to inform the LZ SR1 background model. Present radon mitigation strategies have proven vital for minimising the radon activity of current detectors, but can be strengthened by cold radon emanation assays, due to the expected suppression of radon diffusion in some materials at cold temperatures. The world’s first Cold Radon Emanation Facility (CREF) is being developed as part of the UK’s STFC R&D program Xenon Futures, working towards the development of the third generation dark matter experiment. This thesis will describe the commissioning of CREF and the first comparative high and low temperature assays of radon emanation for materials of interest. Electrons and photons following large interactions in the TPC induce accidental events, which have emerged as a dominant background for ultra-low energy interactions, including WIMPs and 8B searches. The veto built to remove these events in SR1 resulted in a livetime loss of ∼ 30%. Improvements made to this veto through re-optimisation and behaviour tracking by the author have allowed the LZ to reclaim ∼ 20– 25% of its exposure time. This is the largest cost and time effective benefit achieved since the delivery of the SR1 results, with a major impact on the detector sensitivity.

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