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LZ commissioning and background studies

Angelides, Nicolas; (2022) LZ commissioning and background studies. Doctoral thesis (Ph.D), UCL (University College London). Green open access

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Abstract

What is our Universe made of? Dark matter is a hypothetical non-luminous form of matter whose existence is inferred from a rich collection of astrophysical observations. Its gravitational effects are observed across many scales while its behaviour implies properties not found in known forms of matter and the standard model of particles. The Weakly Interacting Massive Particle (WIMP) is a popular simplified dark matter candidate model that agrees with observations while predicting weak-scale interactions with normal matter. The LZ experiment creates conditions under which the interaction between dark matter and normal matter may be detected. The main detector is capable of discriminating and reconstructing interactions with target nuclei within its unprecedented seven tonne liquid xenon active region. Reducing background interactions from radioactive sources creates a very clean observable space within which WIMP interactions may be found. Lower detector backgrounds improve the experiment’s reach to smaller WIMP interaction cross-sections. The LZ experiment is projected to exclude the spin-independent WIMP-nucleon cross-section down to 1.4×10−48cm2 for a 40 GeV WIMP. One of the most important contributions to background interactions comes from decays within the 238U series, but the 232Th and 227Ac series are still relevant. This thesis lays out the first measurement of decays from these series taken during the commissioning period of the experiment when the detector was operated with cold xenon gas. This is the earliest cryogenic measurement of the assembled detector’s backgrounds, made possible through the development of a custom trigger. The activity of 222Rn decays was found to be 3.39 ± 0.13 µHz/kg (assuming 7 tonnes liquid Xenon), demonstrating a factor of two suppression from room temperature emanation results. A further suppression in activity is expected with liquid xenon and the engagement of the integrated radon reduction system. The activity of 220Rn, 219Rn and 210Po plate-out (surface accumulation of parent 210Pb during assembly), was also measured and found to be in agreement with projections for the detector.

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: LZ commissioning and background studies
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Copyright © The Author 2022. Original content in this thesis is licensed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC 4.0) Licence (https://creativecommons.org/licenses/by-nc/4.0/). Any third-party copyright material present remains the property of its respective owner(s) and is licensed under its existing terms. Access may initially be restricted at the author’s request.
UCL classification: UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Physics and Astronomy
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL
URI: https://discovery.ucl.ac.uk/id/eprint/10143162
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