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Title: Dark matter searches and study of electrode design in LUX and LZ
Author: Bailey, Adam
ISNI:       0000 0004 5989 7975
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2016
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There is substantial evidence that over 80% of matter in the universe is dark matter - which is non-baryonic in nature and is thought to be composed of a new, slow-moving, stable particle not found in the Standard Model of Particle Physics. Its presence is inferred from gravitational effects on luminous matter from several independent observations, from the galactic to the cosmological scale. Weakly Interacting Massive Particles (WIMPs) are the leading candidate, which can explain all of the observed effects. LUX and LZ are dual-phase xenon time projection chambers (TPC), aiming to observe scattering of WIMPs from xenon nuclei. LUX has an active mass of 250 kg of liquid xenon, and took data at the Sanford Underground Research Facility in Lead, South Dakota, between 2013 and 2016. The first WIMP search run of 85 live days in 2013 set world-leading exclusion limits on the spin-independent WIMP-nucleon cross section. This was improved by a reanalysis of those data, and subsequently by a new run yielding 332 live days, which set a minimum exclusion limit of 2.2 x 10⁻⁴⁶ cm² for a 50 GeV WIMP (90% CL). In addition, the most stringent limit to date on the spin-dependent WIMP-neutron scattering cross section comes from the reanalysis of the 2013 dataset, with a minimum exclusion of 9.4 x 10⁻⁴¹ cm² for a 33 GeV WIMP. LZ is a next generation experiment with a 7 tonne active mass to be deployed in the same location as LUX, expected to be 100 times more sensitive. Work presented in this thesis includes analysis of the 2013 LUX search data to produce the spin-dependent results, evaluating the detector response using a tritium β-source, and determining the 85Kr background from data. A study was carried out on spurious electron emission phenomena from thin cathodic wires under high electric fields, using LUX engineering data where the grid voltages were increased above nominal operating values; this led to new insights into the microscopic breakdown mechanisms which have affected these (and other) TPC detectors for decades. The detailed understanding of the electroluminescence response gained in LUX was applied to the design of the LZ electroluminescence region; detailed simulation work of electrode geometry was performed to assess the performance of several candidate designs.
Supervisor: Araújo, Henrique Sponsor: Science and Technology Facilities Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral