PMT studies and loop antenna development for the LZ dark matter experiment

The nature of dark matter remains one of the biggest mysteries of the universe. Extensions to the Standard Model of particle physics provide potential candidates for dark matter. Such dark matter particles can be searched for using direct detection experiments. LUX-ZEPLIN (LZ) is a next-generation search for dark matter currently under construction at SURF, South Dakota. LZ employs a two-phase, liquid xenon time projection chamber (LXe TPC). A dark matter particle would scatter of a xenon nucleus, which causes scintillation and electroluminescence light signals. The light is detected by Photomultiplier Tubes (PMTs) on the top and bottom of the detector. The sensitivity projection for a 1000 live day run with 5.6 tonne fiducial mass is 1.6 x 10-48 cm2for a 40 GeV/c2 mass WIMP at 90% confidence level. The first part of this thesis is concerned with the precise modelling of PMT properties and their impact on detector performance. To optimise sensitivity to WIMP masses of less than 10 GeV, it is essential to understand the single photon PMT response in detail. A simulation model, including undersized PMT pulses, is developed based on data taken with spare LZ PMTs. Variation in the PMT single photon response impacts detection efficiencies, energy resolution and position reconstruction. The improved PMT simulation is used to assess the impact of PMT failures on LZ signal detection efficiency, signal to background discrimination and ultimately LZ sensitivity. High voltage discharges, accompanied by sparks, are a potential danger to the PMTs and a known problem in liquid noble gas detectors. The second part of this thesis discusses the development of the first dedicated high voltage discharge monitoring sensor in a LXe TPC. The loop antenna provides useful information about the electromagnetic environment of the detector and monitors for such discharges.