We develop two new methods to measure the structure of matter around the Milky Way using stellar tidal streams from disrupting dwarf galaxies and globular clusters. The dark matter halo of the Milky Way is expected to be triaxial and filled with substructure, but measurements of the shape and profile of dark matter around the Galaxy are highly uncertain and often contradictory. We demonstrate that kinematic data from near-future surveys for stellar streams or shells produced by tidal disruption of stellar systems around the Milky Way will provide precise measures of the gravitational potential to test these predictions. We develop a probabilistic method for inferring the Galactic potential with tidal streams based on the idea that the stream stars were once close in phase space and test this method on synthetic datasets generated from N-body simulations of satellite disruption with observational uncertainties chosen to mimic current and near-future surveys of various stars. We find that with just four well-measured stream stars, we can infer properties of a triaxial potential with precisions of order 5--7 percent. We then demonstrate that, if the Milky Way's dark matter halo is triaxial and is not fully integrable (as is expected), an appreciable fraction of orbits will be chaotic. We examine the influence of chaos on the phase-space morphology of cold tidal streams and show that streams even in weakly chaotic regions look very different from those in regular regions. We discuss the implications of this fact given that we see several long, thin streams in the Galactic halo; our results suggest that long, cold streams around our Galaxy must exist only on regular (or very nearly regular) orbits and potentially provide a map of the regular regions of the Milky Way potential. We then apply this understanding of stream formation along chaotic orbits to the interpretation of a newly-discovered, puzzling stellar stream near the Galactic bulge. We conclude that the morphology of this stream is consistent with forming along chaotic orbits due to the presence of the time-dependent Galactic bar. These results are encouraging for the eventual goal of using flexible, time-dependent potential models combined with larger data sets to unravel the detailed shape of the dark matter distribution around the Milky Way.

Over much of the last century evidence has been building for a new component of our universe that interacts primarily through gravitation. Known as cold dark matter, this non-luminous source is predicted to constitute 83% of matter and 26% of mass-energy in the universe. Experiments are currently searching for dark matter via its possible creation in particle colliders, annihilation in high-density regions of the universe, and interactions with Standard Model particles. So far dark matter has eluded detection so its composition and properties remain a mystery. Weakly interacting massive particles (WIMPs) are hypothetical elementary particles that interact on the scale of the weak nuclear force. They naturally satisfy predictions from extensions of the Standard Model, and are one of the most favored dark matter candidates. A number of direct detection experiments dedicated to measuring their predicted interactions with atomic nuclei have been constructed over the last 25 years. Liquid xenon dual phase time projection chambers (TPCs) have led the field for spin-independent WIMP searches at WIMP masses of >10 GeV/c^2 for most of the last decade. XENON1T is the first tonne-scale TPC, and with 278.8 days of dark matter data has set the strictest limits on WIMP-nucleon interaction cross sections above WIMP masses of 6 GeV/c^2, with a minimum of 4.1 x10^{-47} cm^2 at 30 GeV/c^2. XENON1T and the analysis that led to this result are discussed, with an emphasis on electronic and nuclear recoil calibration fits, which help discriminate between background and WIMP-like events. Interactions in liquid xenon produce light and charge that are measured in TPCs. These signals are attenuated by electronegative impurities including O_2 and H_2O, which are homogeneously distributed throughout the liquid xenon. The decrease in observables enlarges the uncertainty in our analysis, and can decrease our sensitivity. Methods on measuring the charge loss are presented, and a physics model that describes the behavior of the electronegative impurity concentration over the lifetime of XENON1T is derived. The model is shown to successfully explain the more than two years of data.

An abundance of cosmological evidence suggests that cold dark matter exists and makes up 83% of the matter in the universe. At the same time, this dark matter has eluded direct detection and its identity remains a mystery. Many large international collaborations are actively searching for dark matter through its potential annihilation in high-density regions of the universe, its creation in particle accelerators, and its interaction with Standard Model particles in low-background detectors. One of the most promising dark matter candidates is the weakly interacting massive particle (WIMP) which falls naturally out of extensions of the Standard Model. A variety of detectors have been employed in the search for WIMPs, which are expected to scatter with atomic nuclei, yet none have been more successful than dual-phase liquid xenon time projection chambers (TPCs). The first ton-scale liquid xenon TPC, XENON1T, began operating in 2016 and with only 34.2 days of data has set the most strict limits on the WIMP-nucleon interaction cross sections for WIMP masses above 10 GeV/c^2, with a minimum of 7.7 × 10−47 cm^2 for 35 GeV/c^2 WIMPs. One of the major keys to success for liquid xenon TPCs is our understanding of interactions in the medium through myriad measurements. Given that the expected WIMP scattering rate increases with decreasing interaction energy, there has been more focus in recent years in pushing our understanding of interactions in liquid xenon to lower energies. Additionally, as liquid xenon TPCs operate with a large electric field in the medium, an effort has been made to understand how the signal response of xenon changes as a function of the applied electric field. In this thesis, I describe the details of XENON1T, its calibration and characterization, with a special emphasis on the electronic and nuclear recoil calibrations, and the inaugural WIMP search of XENON1T. I then discuss a dedicated measurement, made in the calibration-optimized liquid xenon TPC neriX, of the signal response of low energy nuclear recoils in liquid xenon at electric fields relevant to the dark matter search. The measurements of signal response in XENON1T and neriX were performed using an analysis framework that I developed to allow a more sophisticated examination of recoil responses using GPU-accelerated simulations.

The advent of laser cooling and optical manipulation for atomic samples revolutionized atomic physics in 1990’s, allowing the creation of new phases of matter, more accurate atomic clocks, and enabling leading candidates for the first functional quantum computer. This could not have been predicted at the time, and is a testament to the value of fundamental research for its own sake. These same laser cooling techniques are now being applied to simple molecular systems with the same revolutionary potential. In this thesis, I will present a range of experiments exploring these schemes in a new class of molecules, the diatomic alkaline earth hydrides. We present the creation and characterization of a bright beam of cold barium hydride molecules, high precision spectroscopy of these samples, as well as optical deflection and transverse cooling. This represents the first laser cooling of a Hydride molecule. This is a crucial step towards the creation of new cold molecular samples for a variety of scientific applications. In the final chapter, I will change gears, and introduce new ideas for the detection of scalar field dark matter. While this variety of dark matter is typically searched for using atomic clocks, I will show that the same coupling also leads to anomalous acceleration of test masses. This acceleration would be detectable using both a network of precision acceleration sensors known as the IGETS network, and by the LIGO observatory. This new technique will compliment existing search strategies, and has higher sensitivity for a wide region of parameter space.