Browsing by Subject "Dark matter"

The Standard model process of Coherent Elastic Neutrino-Nucleus Scattering (CEvNS), which was first predicted by Freedman in 1974, has recently been observed by the COHERENT collaboration on CsI and liquid argon targets. The result is a new way to build a compact neutrino detector which unlocks new channels to test the Standard Model. A semiconductor germanium detector, a technology that has been developed by many dark matter direct detection experiments due to its excellent energy resolution and low-energy thresholds, will also be deployed to ORNL in order to detect CEvNS as part of the next phase of the COHERENT experiment. One of the challenges is to understand the signature of neutrino-induced low-energy nuclear recoils in germanium. A measurement was carried out at the Triangle Universities Nuclear Laboratory (TUNL) to characterize the it response to low-energy nuclear recoils. A quenching factor of 14-20% for nuclear recoil energies between 0.8-4.9 keV in Ge was established. A long predicted smearing effect due to quenching was observed for the first time and estimated to be 0.024 at ~2 keVnr. Finally, the impact of this effect and the quenching factor on the expected CEvNS spectrum of the future Ge deployment is presented.

Einstein's General Theory of Relativity sits among the pillars of modern physics as the means by which we describe the universe across an enormous range of scales. This theory has furnished our most robust understanding of the origins of the universe, the dynamics of astronomical objects, and the fundamental structure of space and time. For all of general relativity's successes, however, a wide array of deep questions remain. Its sophisticated mathematical structure renders foundational questions surrounding the extent to which the theory is well-posed difficult to answer (and indeed, difficult to ask), and consistent systematic discrepancies between the universe's dynamics and what the theory leads us to expect given our knowledge of the structure of matter leave us puzzling over which of general relativity and particle physics is more incomplete.

This thesis seeks to explore a small cross-section of the fundamental challenges faced by general relativity through two distinct avenues. The first is an investigation of the cosmological properties of scalar field dark matter, often informed by the fact that it may arise through a minor geometric adjustment to the core structure of the theory. The novel cosmological phenomena under consideration primarily include a dark-matter dominated regime in the early universe and a modification to the standard gravitational redshift, and we generally find that (though they are not ruled out) there is little compelling evidence for either amongst the empirical probes considered herein, namely the anisotropies in the cosmic microwave background radiation as measured by the Planck collaboration and a six-year time-domain survey of spectra across many astronomical sources completed by the Anglo-Australian Telescope. The second is a reflection on both the challenge and posing of the Weak Cosmic Censorship Conjecture, the problem of whether singularities in general relativity must generically reside within black holes. We demonstrate that violating singularities are generic within a particular class of spherically symmetric spacetimes, the Vaidya spacetimes, and this reflection leads us to the development of a novel characterization of the phenomenon of black holes, utilized to formulate a more comprehensive rigorous statement of weak cosmic censorship.

Neutrinos have long been considered a powerful tool for exploring physics beyond the standard model, and have been recognized as having applications in nuclear reactor monitoring and non-proliferation efforts. In particular, there is interest on the part of both the physics and nuclear security communities in a discrete neutrino detector; however, the experimental difficulties associated with detecting neutrinos in a high background environment have hampered past efforts, forcing experiments underground. I discuss my work on a variety of novel neutrino technologies meant to overcome such difficulties. These include the design of a compact optical time projection chamber (TPC) capable of reconstructing inverse beta decays, work on the CHANDLER detector technology systems, the first measurement of nuclear quenching effects in Cerium Bromide scintillator, measurements of nuclear quenching in a gaseous dark matter detector, and a world leading measurement of nuclear quenching in liquid Xenon. Many of these technologies, either singly or in combination, may meet the needs of the nuclear security and dark matter communities and provide a mechanism to reduce backgrounds in fundamental neutrino physics searches.

The question of ``What is Dark Matter?" has been a focus of cosmological research since the turn of the 20th century. Though the composition of Dark Matter is unknown, the existence of Dark Matter is crucial to the modern theory of cosmology. We focus on a theory of Dark Matter referred to as \textit{Scalar Field Wave Dark Matter} (SF$\psi$DM), which has received an increasing amount of interest from the research community since the late 2000s. SF$\psi$DM is a peculiar theory in which Dark Matter is composed of ultralight bosonic particles. As a result, SF$\psi$DM has an astronomically large deBroglie wavelength, generating complicated wave dynamics on the largest cosmological scales.

This thesis focuses on describing the status of SF$\psi$DM theory, SF$\psi$DM halos, and how SF$\psi$DM halos are affected by the wave-like features of the scalar field. In particular, we offer an analysis of galactic rotation curves and how they relate to SF$\psi$DM excited states. This analysis yields a novel model for an observed galactic trend referred to as the Baryonic Tully-Fisher Relation. Furthering this model, we formulate an eigenfunction decomposition which can be used to describe superpositions of excited states.

We examine the Einstein equation coupled to the Klein-Gordon equation for a complex-valued scalar field. These two equations together are known as the Einstein-Klein-Gordon system. In the low-field, non-relativistic limit, the Einstein-Klein-Gordon system reduces to the Poisson-Schrödinger system. We describe the simplest solutions of these systems in spherical symmetry, the spherically symmetric static states, and some scaling properties they obey. We also describe some approximate analytic solutions for these states.

The EKG system underlies a theory of wave dark matter, also known as scalar field dark matter (SFDM), boson star dark matter, and Bose-Einstein condensate (BEC) dark matter. We discuss a possible connection between the theory of wave dark matter and the baryonic Tully-Fisher relation, which is a scaling relation observed to hold for disk galaxies in the universe across many decades in mass. We show how fixing boundary conditions at the edge of the spherically symmetric static states implies Tully-Fisher-like relations for the states. We also catalog other ``scaling conditions'' one can impose on the static states and show that they do not lead to Tully-Fisher-like relations--barring one exception which is already known and which has nothing to do with the specifics of wave dark matter.

We explore a model of dark matter called wave dark matter (also known as scalar field dark matter and boson stars) which has recently been motivated by a new geometric perspective by Bray. Wave dark matter describes dark matter as a scalar field which satisfies the Einstein-Klein-Gordon equations. These equations rely on a fundamental constant Upsilon (also known as the ``mass term'' of the Klein-Gordon equation). Specifically, in this dissertation, we study spherically symmetric wave dark matter and compare these results with observations of dwarf spheroidal galaxies as a first attempt to compare the implications of the theory of wave dark matter with actual observations of dark matter. This includes finding a first estimate of the fundamental constant Upsilon.

In the introductory Chapter 1, we present some preliminary background material to define and motivate the study of wave dark matter and describe some of the properties of dwarf spheroidal galaxies.

In Chapter 2, we present several different ways of describing a spherically symmetric spacetime and the resulting metrics. We then focus our discussion on an especially useful form of the metric of a spherically symmetric spacetime in polar-areal coordinates and its properties. In particular, we show how the metric component functions chosen are extremely compatible with notions in Newtonian mechanics. We also show the monotonicity of the Hawking mass in these coordinates. Finally, we discuss how these coordinates and the metric can be used to solve the spherically symmetric Einstein-Klein-Gordon equations.

In Chapter 3, we explore spherically symmetric solutions to the Einstein-Klein-Gordon equations, the defining equations of wave dark matter, where the scalar field is of the form f(t,r) = exp(i omega t) F(r) for some constant omega in R and complex-valued function F(r). We show that the corresponding metric is static if and only if F(r) = h(r)exp(i a) for some constant a in R and real-valued function h(r). We describe the behavior of the resulting solutions, which are called spherically symmetric static states of wave dark matter. We also describe how, in the low field limit, the parameters defining these static states are related and show that these relationships imply important properties of the static states.

In Chapter 4, we compare the wave dark matter model to observations to obtain a working value of Upsilon. Specifically, we compare the mass profiles of spherically symmetric static states of wave dark matter to the Burkert mass profiles that have been shown by Salucci et al. to predict well the velocity dispersion profiles of the eight classical dwarf spheroidal galaxies. We show that a reasonable working value for the fundamental constant in the wave dark matter model is Upsilon = 50 yr^(-1). We also show that under precise assumptions the value of Upsilon can be bounded above by 1000 yr^(-1).

In order to study non-static solutions of the spherically symmetric Einstein-Klein-Gordon equations, we need to be able to evolve these equations through time numerically. Chapter 5 is concerned with presenting the numerical scheme we will use to solve the spherically symmetric Einstein-Klein-Gordon equations in our future work. We will discuss how to appropriately implement the boundary conditions into the scheme as well as some artificial dissipation. We will also discuss the accuracy and stability of the scheme. Finally, we will present some examples that show the scheme in action.

In Chapter 6, we summarize our results. Finally, Appendix A contains a derivation of the Einstein-Klein-Gordon equations from its corresponding action.