Commissioning of the Outer Detector of the LUX-ZEPLIN experiment and a study of the experiment’s sensitivity to signatures arising from effective field theory operators

Abstract

There is compelling evidence, both astrophysical and cosmological, that the bulk of the mass of the universe is comprised of a non-luminous and near collisionless substance, so-called dark matter. A favoured dark matter candidate is the Weakly Interacting Massive Particle (WIMP), which may be detectable via scattering off of nucleons. The LUX-ZEPLIN (LZ) experiment is a second-generation dual-phase Time Projection Chamber (TPC), which searches for these interactions. The sensitivity of LZ is enhanced by two veto detectors, a liquid xenon Skin detector and a gadolinium-doped liquid scintillator Outer Detector (OD).
To understand the behaviour of the detectors to interactions from dark matter and background signals, large-scale simulations are required. Presented in this work is a solution to overcome the limitations of CPU simulations. The GPU approach explored in this work showed an improvement in the simulation time of optical simulations of 720-times compared to CPU simulations.
In order to make use of the OD for physics analyses, the backgrounds and neutron veto performance need to be understood. The expected backgrounds have been fitted to the observed spectra from data, which showed an elevated rate of 210Po α-decays. This has been linked to radon-progeny plate-out inside of the OD during assembly and installation. The neutron veto efficiency of the OD is measured in this work to be 84.6±1.2%, which is lower than the design requirement of 95%. The decrease is linked to water ingress between the OD and other detectors, reducing the number of neutrons which enter the OD.
Also described in this thesis is the sensitivity of the experiment to signatures arising from Effective Field Theory (EFT) operators. The projected sensitivity of LZ to these operators in an extended region of interest, up to ∽270 keV, is shown to be up to 5-orders of magnitude better than in previous experiments. Analysis in a reduced region of interest (∽70 keV) on the first science run data was also performed. Exclusion limits are set on the coupling of each EFT operator with an improvement upon previous limits of up to 3 orders of magnitude. World-leading limits are set for all operators in at least part of the explored phase-space.

Date of Award	6 Dec 2022
Original language	English
Awarding Institution	University of Bristol
Supervisor	Henning U Flaecher (Supervisor) & Jim Brooke (Supervisor)