This thesis represents a comprehensive study of solar neutrinos spanning three generations of solar neutrino detection techniques. Data from the Sudbury Neutrino Observatory (SNO), a heavy water detector instrumental in demonstrating flavor change in the neutrino sector, is used to set a limit on the lifetime of massive neutrino states within a model where said states can decay. The analysis of SNO data finds a limit of $k_2>8.08 \times 10^{-5}$~s/eV at 90\% confidence for the lifetime of $\nu_2$. Combining this limit with constraints from all other solar neutrino experiments, a new global best limit is found at $k_2>1.92 \times 10^{-3}$~s/eV at 90\% confidence for the lifetime of $\nu_2$.

The upgrade of the SNO detector, SNO+, is currently in the process of migrating to a liquid scintillator target material. Prior to this, SNO+ produced a physics dataset with light water as the target, which was analyzed to extract the flux of $^8$B solar neutrinos. This analysis resulted in a measured flux of $\Phi_{\mathrm{^8B}} = 5.95^{+0.75}_{-0.71}\mathrm{(stat.)}^{+0.28}_{-0.20}\mathrm{(syst.)} \times 10^6\,\,\mathrm{cm}^{-2}\mathrm{s}^{-1}$. Further, this analysis demonstrated SNO+ has achieved very low backgrounds above 6.0~MeV visible energy. To aid in detector calibrations as SNO+ moves from water to scintillator, the design and evaluation of a Cherenkov optical calibration source is presented.

With an eye on the future, the capabilities of the proposed Theia experiment are discussed, with particular attention to the novel water-based liquid scintillator target being evaluated for use by that experiment. A small scale experiment to evaluate the prospects of distinguishing scintillation and Cherenkov light in this target is described. Results are presented demonstrating the potential for identification of Cherenkov photons by timing and intensity information for this water-based liquid scintillator, and for a more traditional scintillator, LAB+PPO.

This dissertation presents neutron-based measurements made using the water-Cherenkov SNO detector filled with heavy water, and the scintillator-based SNO+ detector filled with liquid scintillator, as well as lab-scale R&D measurements of water-based liquid scintillator (WbLS), a candidate material for achieving hybrid Cherenkov/scintillation technology. The results are of interest for the design of future detectors, from the perspectives of both background prediction and reconstruction capabilities, as well as improving the understanding of the nucleus and its interactions and the nature of dark matter, and the cosmos at large.

Neutrons produced by cosmic-ray muons in the SNO detector are characterized, and their production rate is measured to be, in units of 10^-4 cm / (g · μ), 7.28 +/- 0.09 (stat.) +1.59/-1.12 (syst.) and 7.30 +/- 0.07 (stat.) +1.40/-1.02 (syst.) in pure deuterium and deuterium loaded with NaCl at 0.02% by weight, respectively. A comparison of high-level observables in the accumulated data and a set of GEANT4-based simulations reveals generally accurate modeling of the production and transport physics in heavy water, but may indicate shortcomings in high-energy interactions with sodium and chlorine nuclei, which warrants further investigation.

A preliminary search for extraterrestrial antineutrinos with SNO+, which would manifest as an excess of tens-of-MeV inverse beta-decay (IBD) events and be indicative of new astrophysical phenomena, potentially shedding light on the nature of dark matter, has yielded no significant signal. Statistical analysis in a Bayesian framework has produced a 90% credible limit on the flux of astrophysical antineutrinos of approximately 10^3 cm^-2 s^-1 MeV^-1, further decreasing as a function energy. Sensitivity projections for an updated analysis on 5 years worth of data indicate that SNO+ would achieve limits comparable to the current world-leading limits, which would be further improved by extensions to reconstruction techniques to adapt to a higher-energy regime.

Also presented is a characterization of the response of both WbLS and LAB+PPO, a conventional scintillator cocktail, to MeV-scale protons via a broad-spectrum neutron beam, and electrons and alpha particles via radioactive sources. The results of this characterization are relevant for measurements made in a low-energy regime, including antineutrinos from nuclear reactors and solar neutrinos, and demonstrate timing-based Cherenkov/scintillation discrimination in electron interactions at the benchtop scale.

Many compelling pieces of indirect evidence pointing to the existence of dark matter. While a confirmed and direct signature of dark matter has yet to be observed, many theoretical models have been developed in an attempt to explain the indirect evidence and to provide phenomenological models that can be tested with targeted experiments. The WIMP is a well-motivated dark matter candidate currently being sought for by several experiments. A variety of detector technologies are utilized, including liquid noble detectors, to look for WIMP scattering as a direct signature of dark matter.

The CLEAN experiment is a proposed single-phase, monolithic, large-scale liquid argon experiment designed to look for high-mass WIMPs. A liquid neon target could be exchanged with the argon target to study solar neutrinos and to test the A2 dependence of a possible dark matter signal. Before scaling up to the multi-tonne scale of the full CLEAN detector, the design philosophy and background rejection capabilities required for the next-generation project are being tested using the MiniCLEAN prototype. As of mid-2018, MiniCLEAN has been constructed at SNOLAB and is currently being filled with natural liquid argon for a dark matter run. Following a short dark matter run, MiniCLEAN will be spiked with elevated levels of 39Ar to test the scaling limits of pulse shape discrimination, the primary method for electronic background rejection. These results will inform existing experiments and the next-generation of large-scale liquid argon detectors.

A good understanding of light propagation is critical for optical experiments such as CLEAN, whose event reconstruction and background rejection relies primarily on scintillation light collection. This work presents two classes of complementary results which are expected to improve the modeling of scintillation light collection in current and future neutrino and dark matter detectors. These are, first, the dependence of the scintillation light time structure (triplet lifetime) and relative light yield of gaseous argon as a function of impurity level and, second, the measurement of several parameters critical to constructing a microphysically-motivated model of tetraphenyl butadiene (TPB) wavelength shifting thin films - a technology which is commonly used in many existing and proposed liquid noble gas experiments.

This dissertation chronicles the role of optical photon-based detection technologies in the past, present, and future of neutrino physics. The initial chapters summarize the history of the field, following chapters explaining key areas of current research. The focus splits then splits, first covering studies with the current-generation, kiloton-scale SNO+ experiment, which has operated with significant amounts of liquid scintillator as its target since 2020. The next sections highlight work undertaken toward the development of a new paradigm known as “hybrid” detection, which aims to benefit from the two optical light emission mechanisms, Cherenkov radiation and scintillation, currently drawn on separately in today’s experiments.

For SNO+, the experiment is described and this work explores the first demonstrations of α and instrumental background rejection on scintillator data, performed using likelihood-ratio-based classification with hit timing. These demonstrations provide powerful tools for a broad range of physics analyses in SNO+. Additionally, an analysis to determine the 8B solar neutrino flux is performed on two datasets, one from when the SNO+ detector was only partially filled with liquid scintillator for an extended period of time due to the COVID-19 pandemic, and one from when the detector was completely full with the final scintillator cocktail for a period of over a year. The measured flux in both periods, [5.13+1.29−1.11(stat.)+0.45−0.53(syst.)]×10^6 cm^−2 s^−1 and [5.74+0.84−0.77(stat.)]×10^6 cm^−2 s^−1 respectively, is consistent with theoretical predictions from leading Standard Solar Models. This gives confidence in the understanding of SNO+’s operations in this period and adds to the family of measurements made of this flux around the community.

Subsequent discussion introduces the hybrid paradigm and outlines the areas where this technology is maturing. This dissertation presents key explorations into the physics potential at large-scales of this technology using well-motivated modeling and reconstruction for the first time. The potential for neutrinoless double beta decay and CNO solar neutrino flux measurements are examined, with capabilities akin to or exceeding state of the art experiments in a range of scenarios. Also presented is the particle identification capability of the novel scintillating medium water-based liquid scintillator based on lab-measured timing and light yield properties, with substantial rejection power identified between α and β signals. These explorations provide a confirmation of the possibilities for hybrid detection and help pave the way for concrete realizations of these technologies at larger scales.