Search for new physics via low-energy electron recoils with the LZ experiment and low background technique enhancements with the Boulby laboratory

Abstract

The fundamental nature of dark matter is one of the greatest mysteries in physics. Compelling evidence indicates that dark matter constitutes 84% of the total matter density in the universe, yet the detection of its constituent remains elusive. The LUX-ZEPLIN (LZ) experiment, located in Sanford Underground Research Facility, employs several tonnes of xenon in a dual-phase time projection chamber to conduct direct searches for dark matter particles. Recently, the LZ experiment set a world-leading limit on interactions between nucleons and weakly interacting massive particles (WIMPs), one of the most promising dark matter candidates.

To ensure the detector’s stability and maintain data integrity, the LZ experiment implements extensive monitoring and quality assurance measures including the development of online monitoring tools, routine calibrations, and a structured shift system across collaborating institutes. This author’s contributions include assuring PMT quality, conducting grid testing campaign and calibration tasks, and serving as a member of the PMT supervision team.

This thesis investigates the sensitivity and versatility of the LZ detector in extending its scientific programme beyond WIMPs to search for other novel physics from the hidden sector, such as axion-like particles, hidden photons, and mirror dark matter. All dark matter models discussed are expected to produce electron recoil (ER) interactions in the liquid xenon chamber and generating signals in the low-energy ER channels. The results rule out the areas of allowed parameter space for these signals using the LZ’s experiment first exposure of 60 live days and a fiducial mass of 5.5 tonnes. Future science-runs from LZ with higher exposure will further improve the sensitivities of these searches. A crucial aspect in the field of direct detection is the continuous improvement of detector sensitivity. This often involves developing material assaying techniques to reduce background and providing accurate characterisations of residual radiocontaminants in the detector. The Boulby UnderGround Screening (BUGS) facility is one of the few material screening facilities in the world that support rare-event search experiments such as LZ. Works have been conducted to improve the Boulby Underground Laboratory’s overall assaying capabilities through the addition of the commercially manufactured XIA UltraLo-1800 alpha particle counters. These findings demonstrate how the XIA UltraLo-1800 are able to enhance the material assaying techniques at Boulby. These efforts summarised above, in ruling out allowed parameter space for dark matter models and improving low background techniques for rare event searches, aspire to unravel the characteristics of dark matter.