Dark Matter Search with the PICO-40L Bubble Chamber

Abstract

The identity of dark matter remains one of the largest unsolved mysteries in modern physics, with international efforts going towards dark matter direct detection. The PICO collaboration, which operates bubble chambers in the search for WIMP-like dark matter, is currently operating the PICO-40L experiment at SNOLAB in Sudbury, Ontario, Canada. PICO-40L, the first large-scale implementation of the so-called "right side up" design, has been constructed over the past several years and has recently to gather open data for the characterization of backgrounds. Through the work undertaken in this thesis, PICO-40L has become an extremely stable bubble chamber. The fine pressure and temperature control offered by the hydraulic and thermal systems have delivered a chamber that may be maintained at a very stable threshold. During the commissioning phase, the tools developed for the analysis of the data were refined, including the signals from the piezoelectric transducers, the fast pressure data from the Dytrans, and the 3D position reconstruction toolchain. Through the refinement of these tools in the course of this thesis work, the acoustic parameter has been improved to the point that the decays of 222Rn and 218Po, two of the decays in the uranium chain commonly observed in PICO detectors, are becoming separable. With a comparable fiducial volume to its predecessor, PICO-40L will provide a modest improvement in dark matter limits compared to PICO-60, the previous bubble chamber operated by the PICO collaboration at SNOLAB. However, it serves as proof of the operational stability of the RSU design, which will be employed in the next-generation PICO-500 detector. PICO-500 will benefit from the use of similar instrumentation to that of PICO-40L, and will surpass the spin-independent and spin-dependent WIMP exclusion limits set by PICO-60 and PICO-40L by more than an order of magnitude. The outcome of this thesis, with the transformation of the PICO-40L bubble chamber from a concept to a highly performant dark matter detector, bodes extremely well for the upcoming generation of the PICO bubble chamber programme and the potential for a definitive discovery of dark matter in the future.