Department of Physics, Engineering Physics and Astronomy Graduate Theses
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Item Are Gas-rich Field Ultra-diffuse Galaxies and Dwarf Galaxies Distinct Populations?(2024-09-12) Motiwala, Khadeejah; Physics, Engineering Physics and Astronomy; Spekkens, KristineOver the past decade, significant developments in instrumentation and image searching techniques have uncovered thousands of low surface brightness objects that were previously uncatalogued. Among these are ultra-diffuse galaxies (UDGs) - objects that represent the extremes of galaxy properties. UDGs have stellar content similar to classical dwarf galaxies, but physical sizes more akin to Milky Way-type galaxies; as such, several theories for how UDGs may form differently from dwarfs have been proposed. In this thesis, we aim to constrain the different formation mechanisms in two ways. First, we compile and present the largest catalogue of optically-selected field UDGs and dwarfs with distance measurements in the Systematically Measuring Ultra-Diffuse Galaxies (SMUDGes) catalogue. We compare the UDGs and dwarfs in SMUDGes to investigate whether UDGs are a distinct population. Second, we compare the SMUDGes observations with two state-of-the-art cosmological simulations: NIHAO (Numerical Investigation of a Hundred Astrophysical Objects), which forms UDGs through bursts of star formation at early times and Romulus25, which forms UDGs from major mergers. Although formation scenarios for UDGs with these simulations are remarkably different, the present-day, global properties of the simulated galaxies are consistent with our observed sample. Furthermore, in both simulations and observations, we find no distinct difference between the UDGs and classical dwarf populations within the gas-richness vs size parameter space. The results presented in this work include the first detailed study of gas-rich UDGs in both observations and simulations.Item The Energy Resolution of the SNO+ Detector in Liquid Scintillator Phase and Implications for Double Beta Sensitivity(2024-09-05) Dehghani, Rayhaneh; Physics, Engineering Physics and Astronomy; Wright, AlexanderSNO+ is a multi-purpose liquid scintillator detector that ultimately aims to detect the hypothesized neutrinoless double beta decay (0νββ) through loading of 130Te as the target isotope. To this day the discovery of the neutrino mass and neutrino oscillations is the only proof of physics beyond the Standard Model. The detection of the rare 0νββ decay would revolutionize the field of physics and alter our understanding of the world. To perform such an experiment, the background rates must be extremely low in order to carry out the search for this signal in its energy region-of-interest. SNO+ is situated 2 km underground, in Creighton mine, Sudbury, ON. naturally shielded from cosmic radiation that otherwise would be extremely challenging to get rid of. One of the dominant systematic uncertainties in double beta decay analysis is expected to be the understanding of the energy resolution of the detector. In order to quantify whether the current understanding of the energy resolution is sufficient, a pure selection of mono-energetic 214Po events was collected utilizing 214Bi-214Po coincidences. A comparison of data and Monte Carlo simulation of the 214Po events yielded an estimate of the energy resolution systematic, which was then applied in sensitivity studies using fake datasets to evaluate its impact on the neutrinoless double beta decay sensitivity. Based on the best knowledge of the background in the detector in the liquid scintillator phase, this analysis developed a background model concerning the most contributing background sources in the region-of-interest for 0νββ decay to investigate the sensitivity levels of SNO+ at the current stage. These studies reveal that continued improvement in our understanding of the energy resolution is required.Item Asymmetric dark matter in main-sequence stars: interactions with electrons versus nucleons(2024-09-05) Beram, Stephanie; Physics, Engineering Physics and Astronomy; Vincent, AaronIn this study, we investigate the influence of asymmetric dark matter (ADM) on stellar structure and evolution through its interactions with electrons and nucleons. We examine velocity-dependent and momentum-dependent scattering cross-sections for n = 0, 1, or 2, where v_rel is the DM-target relative velocity and q is the momentum transferred. Comparing DM capture and energy transport in the present-day Sun, our results show that for DM masses over 1 GeV, the peak luminosity of DM-nucleon interactions is consistently two orders of magnitude higher than that of DM-electron interactions. Utilizing MESA for stellar modeling, we extend our analysis to other main-sequence stars, emphasizing the importance of convective core stars. We demonstrate how DM-electron and DM-nucleon interactions can eliminate convective cores, reduce core temperatures, and increase core hydrogen burning rates, significantly altering stellar structure and evolution. Furthermore, we explore the impact of constant ADM interactions on the subgiant star KIC 8228742 and its asteroseismic observables, performing stellar calibration across the DM parameter space to ensure consistency with observational data. By using frequency separation ratios, obtained from the asteroseimic frequency spectrum, to evaluate the fit, we find that for certain DM-electron interactions that eliminate the convective core, the resulting model can achieve up to a 4 sigma improvement over no-DM models.Item Studies of machine learning for event reconstruction in the SNO+ detector and electronic noise removal in p-type point contact high purity germanium detectors(2024-07-31) Anderson, Mark; Physics, Engineering Physics and Astronomy; Martin, RyanThis dissertation presents two distinct topics. Both focus on the development and application of neural networks and deep learning-based methods to rare event searches in physics, specifically neutrinoless double-beta decay. In the first project, a new method for event vertex reconstruction is developed for SNO+ — a large-scale, liquid scintillator-based, multi-purpose neutrino experiment located at SNOLAB in Sudbury, Ontario, Canada. Several studies are conducted to demonstrate its performance in comparison to traditional maximum likelihood reconstruction techniques, as well as its potential to increase the sensitivity of SNO+ to neutrinoless double-beta decay. In the second project, a deep fully convolutional autoencoder is developed and applied to denoise pulses collected from a p-type point contact high purity germanium detector located at Queen's University in Kingston, Ontario, Canada and similar to the germanium detectors used in the arrays of large-scale experiments. It is shown through multiple analyses that denoising using these methods preserves the underlying pulse shape while simultaneously allowing for improvements in the energy resolution and background discrimination power in some circumstances. Detection of the hypothetical neutrinoless double-beta decay could answer long-standing questions in physics and provide a better understanding of the Universe. As such, numerous experiments across the world are running, or under development, to search for this process. While the tools introduced here are applied to a particular liquid scintillator detector and p-type point contact germanium detector, they are broadly applicable to other experimental setups and detection technologies in addition to the specific ones utilized for each project. Furthermore, these tools can be employed to improve the sensitivity of experiments searching for other rare events, such as dark matter, using similar principles. The flexibility and straightforward transfer of these methods are discussed and some ongoing and future work is highlighted. This research is thus relevant both to and beyond the entire rare event search community and has the potential to widely improve analysis techniques, especially in light of the growing size and rates of data collection from modern particle physics experiments.Item Light collection as a veto for PICO and SBC dark matter searches(2024-07-29) Hawley Herrera, Hector; Physics, Engineering Physics and Astronomy; Clark, KenBubble chambers are a promising technology for dark matter detection that use a superheated fluid as the detection medium, where only point-like energy depositions can initiate a bubble nucleation event. PICO and the Scintillating Bubble Chamber (SBC) collaborations use bubble chambers with the goal of detecting WIMP-like dark matter ($\sim$1-100~GeV). PICO uses fluorocarbons as the active fluid, while SBC uses liquid noble elements. PICO-500 is a 250-litre C$_3$F$_8$ bubble chamber located in the Cube Hall at SNOLAB. Muon-induced backgrounds will significantly contribute to the background budget assuming the detector meets its other background goals. The Cherenkov radiation generated by the muons will be used to veto muon-induced backgrounds. The PICO-500 muon veto consists of 48 R1408 photomultiplier tubes (PMTs), a water tank with an average radius of 18’-4.8" and height of 25’-10.5", and 12 LEDs. Geant4 and electronics simulations were developed to determine the efficacy of the muon veto and estimate the impact of noise in the system. These simulations were used to find a compromise between several conflicting components (PMT positions, bill of materials, electronics, etc.). 5,100$\pm$1,300 muons per year are expected to interact with the PICO-500 muon veto. Under the worst-case scenario, the muon veto is expected to have an efficiency of 99.71\% (or $15\pm4$ missed muons per year), which can be increased depending on PICO-500 dead-time requirements. SBC-LAr10 is a 10~kg bubble chamber filled with liquid argon doped with xenon installed in the MINOS tunnel at Fermilab. The use of liquid noble elements introduces scintillation to the data, which enables vetoing capabilities that previous bubble chambers do not have. The generated light will be collected using 32 UV-sensitive Hamamatsu silicon photomultipliers (SiPMs). A novel characterization technique was developed using dark count data, avoiding the requirement of a single-photon source, and it was compared with the more conventional current vs. voltage characterization. The SiPM parameters (breakdown voltage, single photon gain, dark count rate, and others) were estimated to relative accuracies of approximately 0.5\% or Poissonian limited values (as in the case of dark count rate). The extracted temperature coefficients will aid SBC-LAr10 implementation and future experiments using SiPMs.