Browsing by Subject "Dark Matter"

Now showing 1 - 6 of 6
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    Analytical and numerical studies of dark matter halos
    (2008-12) Austin, Crystal Gayle
    This dissertation focuses on the evolution and structure of dark matter halos of galaxies, groups and clusters of galaxies. I explore the dependence of the final halo’s properties on the initial conditions and the physical processes that guide the halo to equilibrium, with special focus on the power-law nature of the ρ/σ3 profile, where ρ is the density profile and σ is the velocity dispersion profile. As the astronomy community does not yet fully understand these processes, this research expands our understanding of collisionless, gravitationally-interacting systems. In the initial chapters, I study the collisionless semi-analytic halo simulations and show that the final properties are sensitive to the initial conditions, such as the powerspectra filtering scale, the secondary velocities’ magnitudes and directions, and the accretion rate. The general conclusions are that semi-analytic halos are in hydrostatic equilibrium and have a power-law ρ/σ3 profile. If there were discontinuities in the initial conditions, the power-law feature in ρ/σ3 breaks. Because of this, hydrostatic equilibrium is a less restrictive condition than the ρ/σ3 profile. These halos can recover from moderate discontinuities by either correcting a single profile by sacrificing other quantities or by sufficient post-accretion. Finally, I compare collisionless semi-analytic and N-body simulations directly. This novel comparison is useful because these techniques use different physics to collapse the proto-halo. The physical differences between these two methods are used to determine causes of the final halo profiles. Specifically, I find the NFW density profile and power-law ρ/σ3 are due to the slow rate of evolution, which is determined from the initial conditions and cosmology. The density slope-velocity anisotropy relationship is dependent, rather, on the physical processes (notably the radial orbit instability) and three-dimensional evolution used to collapse the proto-halos. We also find that the slow-evolution halos do not undergo violent relaxation (large changes in the global potential). Thus we suggest that slow, collisionless relaxation is responsible for creating the power-law feature ρ/σ3.
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    Effective Field Theory Analysis and Active Neutron Veto Design for the Cryogenic Dark Matter Search
    (2018-05) Rogers, Hannah
    Astrophysical measurements and cosmological predictions suggest the exists of a large amount of matter in the universe that does not interact via electromagnetic forces. This non-luminescent matter, known dark matter, exists in halos that encompass and are within galaxies, including the Milky Way. Therefore, dark matter particles should should be directly detectable by experiments on Earth, such as the Super Cryogenic Dark Matter Search (SuperCDMS). Dark matter is assumed to be low mass (< 100 GeV/c2) and interact via the weak force using either a spin-independent or spin-dependent coupling. However, making incorrect assumptions about dark matter interactions can lead to misleading results. Because interactions with dark matter particles are rare, direct detection experiments must be able to shield for or reject backgrounds to very low levels. Low energy neutron backgrounds that make it to the detectors are especially dangerous, because they cannot be easily distinguished from the expected dark matter signal. Scintillator doped with a high neutron-capture cross-section material can be used to detect neutrons via their resulting gamma rays. Examples of such detectors using liquid scintillator have been successfully used in past high-energy physics (HEP) experiments. However, a liquid scintillator can leak and is not as amenable to modular or complex shapes as a solid scintillator. The light outputs and efficiencies of gadolinium-loaded polystyrene-based scintillators have been explored using a wide variety of gadolinium compounds with varying concentrations. Collection strategies using a wavelength shift- ing (WLS) fiber and silicon photomultipliers (SiPMs) were also evaluated as a possible neutron veto for an upgrade to SuperCDMS SNOLAB. The scattering of dark matter particles off nuclei in direct detection experiments can be described in terms of a multidimensional effective field theory (EFT). A new systematic analysis technique is developed using the EFT approach and Bayesian inference methods to exploit, when possible, the energy-dependent information of the detected events, experimental efficiencies, and backgrounds. Highly dimensional likelihoods are calculated over the mass of the weakly interacting massive particle (WIMP) and multiple EFT coupling coefficients, which can then be used to set limits on these parameters and choose models (EFT operators) that best fit the direct detection data. Expanding the parameter space beyond the standard spin-independent isoscalar cross section and WIMP mass reduces tensions between previously published experiments. Combining these experiments to form a single joint likelihood leads to stronger limits than when each experiment is considered on its own. Simulations using two nonstandard operators (O3 and O8) are used to test the proposed analysis technique in up to five dimensions and demonstrate the importance of using multiple likelihood projections when determining constraints on WIMP mass and EFT coupling coefficients. In particular, this shows that an explicit momentum dependence in dark matter scattering can be identified. CDMSlite Run 2 was a search for Weakly Interacting Massive Particles (WIMPs) with a cryogenic 600 g Germanium detector operated deep underground. It was operated in a mode optimizing sensitivity to WIMPs of relatively low mass, 2 - 20 GeV, while sacrificing background rejection. An EFT analysis of CDMSlite Run 2 data from SuperCDMS Soudan is presented here. A binned likelihood Bayesian analysis was performed on the data, optimizing over the parameters of EFT interactions and the recoil energy spectra due to the dominant Compton scattering and tritium backgrounds. Recoil energy regions within 5σ of known activation peaks were removed from the analysis. The Bayesian evidences of the resulting likelihoods show that CDMSlite Run 2 data is entirely consistent with the background models with no EFT interaction necessary. Upper limits on the WIMP mass and coupling coefficients amplitudes and phases are presented for each EFT operator.
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    Holographic and Composite Models of the Axion
    (2021-07) Nguyen, Minh
    The axion, first proposed to solve the absence of a neutron electric dipole moment, is also an excellent dark matter candidate. If the axion is composite, it must be bound by a novel strong interaction. Some strongly-coupled models, mathematically intractable until now, are recently discovered to be calculable through a mapping onto 5-dimensional gravity. We propose a series of composite and extra dimensional axion models. A basic 5-dimensional model first incorporates the known forces while keeping the axion consistent with gravitational corrections. More sophisticated extensions address other problems in particle physics unrelated to the axion, providing nontrivial predictions that can be tested experimentally. We further propose a composite model that incorporates both the Higgs boson and the axion. By enlarging the color gauge group, we provide a consistent procedure to raise the axion masses as high as TeV collider energy scales, further motivating experimental searches for axion-like particles.
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    Low-Mass Dark Matter Search Results and Radiogenic Backgrounds for the Cryogenic Dark Matter Search
    (2016-12) Pepin, Mark
    An ever-increasing amount of evidence suggests that approximately one quarter of the energy in the universe is composed of some non-luminous, and hitherto unknown, “dark matter”. Physicists from numerous sub-fields have been working on and trying to solve the dark matter problem for decades. The common solution is the existence of some new type of elementary particle with particular focus on weakly interacting massive particles (WIMPs). One avenue of dark matter research is to create an extremely sensitive particle detector with the goal of directly observing the interaction of WIMPs with standard matter. The Cryogenic Dark Matter Search (CDMS) project operated at the Soudan Underground Laboratory from 2003–2015, under the CDMS II and SuperCDMS Soudan experiments, with this goal of directly detecting dark matter. The next installation, SuperCDMS SNOLAB, is planned for near-future operation. The reason the dark-matter particle has not yet been observed in traditional particle physics experiments is that it must have very small cross sections, thus making such interactions extremely rare. In order to identify these rare events in the presence of a background of known particles and interactions, direct detection experiments employ various types and amounts of shielding to prevent known backgrounds from reaching the instrumented detector(s). CDMS utilized various γ and neutron shielding to such an effect that the shielding, and other experimental components, themselves were sources of background. These radiogenic backgrounds must be understood to have confidence in any WIMP-search result. For this dissertation, radiogenic background studies and estimates were performed for various analyses covering CDMS II, SuperCDMS Soudan, and SuperCDMS SNOLAB. Lower-mass dark matter has become more prominent in the past few years. The CDMS detectors can be operated in an alternative, higher-biased, mode to decrease their energy thresholds and correspondingly increase their sensitivity to low-mass WIMPs. This is the CDMS low ionization threshold experiment (CDMSlite), which has pushed the frontier at lower WIMP masses. This dissertation describes the second run of CDMSlite at Soudan: its hardware, operations, analysis, and results. The results include new WIMP mass-cross section upper limits on the spin-independent and spin-dependent WIMP-nucleon interactions. Thanks to the lower background and threshold in this run compared to the first CDMSlite run, these limits are the most sensitive in the world below WIMP masses of ∼4 GeV/c^2. This demonstrates also the great promise and utility of the high-voltage operating mode in the SuperCDMS SNOLAB experiment.
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    Lowering Backgrounds and Thresholds in the Search for Light Dark Matter with SuperCDMS
    (2024-01) Nelson, Jack
    Cosmological observations have produced a wealth of evidence which demonstrates that the majority of thematter content in our Universe is “dark”. The identity of this dark matter remains elusive, as none of the members of the standard model of particle physics accurately describe its properties. This has prompted the scientific community to launch a broad search, spanning decades in time, mass and sensitivity, with the goal of detecting and identifying this source of new physics. The next generation of the Super Cryogenic Dark Matter Search (SuperCDMS) is currently under construction deep underground at SNOLAB. The experiment aims to expand the search for dark matter to lower masses (≲ 10 GeV/c2) and greater sensitivities using silicon and germanium detectors by minimizing experimental backgrounds and operating detectors with superb energy resolution. SuperCDMS will accomplish its low projected background in part by deploying a robust shield to protect its detectors from environmental radiation. This dissertation presents the results of simulations which demonstrate the success of the shield design at stopping radiogenic neutrons. The shield will be able to reduce these environmental sources to the point where coherent scattering from solar neutrinos are expected to dominate the nuclear recoil backgrounds. In order to search for such light dark matter masses, SuperCDMS uses sensitive transition edge sensors to measure small energy depositions in the detectors. The ultimate energy resolution of these devices, expected to be < 1 eV, has not yet been realized. This dissertation describes the analysis of a dark matter search performed at the University of Massachusetts Amherst with a prototype detector which uses SuperCDMS style sensors to achieve a baseline energy resolution of 2.3 eV. The results of this search demonstrate sensitivity to dark matter candidates with masses as low as ∼ 25 MeV.
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    SuperCDMS Prototype Detector Design and Testing
    (2017-10) Kennedy, Allison
    A substantial amount of astrophysical evidence indicates that approximately a quarter of all energy in the universe is composed of a nonluminous, and nonbaryonic "dark'' matter. Of the potential dark matter particle candidates, Weakly Interacting Massive Particles, or WIMPs, is particularly well motivated. As a means to directly detect WIMP interactions with baryonic matter, the Cryogenic Dark Matter Search (CDMS) project was established, operating at the Soudan Underground Laboratory from 2003 - 2015, under the CDMS II and SuperCDMS Soudan experiments. CDMS detectors simultaneously measure the ionization and phonon energies of recoil events in Si and Ge crystals kept at cryogenic temperatures in a low-background environment. The ratio of ionization energy to recoil energy serves as a discrimination parameter to separate nuclear recoil events from the electron-recoil background. The next installation, SuperCDMS SNOLAB, is preparing for future operation, with an initial payload of eighteen Ge and six Si, 100 mm diameter, 33 mm thick detectors. Of this initial payload, eight Ge and four Si detectors will operate in a high-voltage (~100 V) mode, which have an increased sensitivity to low-mass WIMPs due to decreased energy thresholds. The SuperCDMS test facility at University of Minnesota aids in the detector R&D and characterization of prototype detectors, as part of the scale-up effort for SuperCDMS SNOLAB. This thesis presents the first full ionization and phonon characterization study of a 100 mm diameter, 33 mm thick prototype Ge detector with interleaved phonon and ionization channels. Measurements include ionization collection efficiency, surface event rejection capabilities, and successful demonstration of nuclear recoil event discrimination. Results indicate that 100 mm diameter, interleaved Ge detectors show potential for use in SuperCDMS SNOLAB. As part of detector R&D, the Minnesota test facility also looks beyond the next stage of SuperCDMS, investigating larger individual detectors as a means to easily scale up the sensitive mass of future searches. This thesis presents the design and initial testing results of a prototype 150 mm diameter, 33 mm thick silicon ionization detector, which is 5.2 times larger than those used in SuperCDMS at Soudan and 2.25 times larger than those planned for use at SuperCDMS SNOLAB. In addition, the detector was operated with contact-free ionization electrodes to minimize bias leakage currents, which can limit operation at high bias voltages. The results show promise for the operation of both large volume silicon detectors and contact-free ionization electrodes for scaling up detector mass and bias.