Pseudo-Nambu Goldstone bosons (pNGBs), such as axion-like particles (ALPs), are a compellingcandidate for cold dark matter. In the following thesis, I will discuss two separate ALP models of cold dark matter. In both models, a spontaneously broken approximate global U(1) symmetry results in the formation of an interlocking network of 1- and 2-dimensional topological defects that is stable unless a bias term is present in the Lagrangian. The annihilation of this system results in the production of ALPs, gravitational waves, and possibly primordial black holes (PBHs). We dub this process “catastrogenesis”, from the Greek word κατ αoτ ρoϕ�η, meaning “annihilation.” The first model I will discuss assumes that the ALPs are stable and therefore constitute some or all of the dark matter. In this case, if produced at temperatures below 100 eV gravitational waves could be detected by future cosmological probes for ALP masses anywhere between 10−16 and 106 eV. The second possibility I discuss is that ALPs could decay into Standard Model products that then thermalize, in which case ALPs do not make up any of the dark matter. However, if PBHs are produced during the last stages of string-wall annihilation they could themselves constitute up to 100% of the dark matter. In addition, produced gravitational waves could be detectable by future interferometers.

Cosmological and Astrophysical observations provide compelling evidences for the existence of dark matter in the universe. One class of dark matter candidates, the Weakly Interacting Massive Particles (WIMPs), has been predicted in many particle physics theories. Direct detection experiments using dual- phase liquid noble element detectors report the best sensitivities to the detection of the dark matter particles. The next generation direct detection experiments using the same technology, are actively been built and expected to give a factor of 100 improvement on the current best sensitivity.

This thesis discusses the measurement of nuclear recoils in a dual-phase liquid argon detector using a bunched neutron beam generated by linear accelerator facility at accelerator laboratory in Notre Dame University. Nuclear recoils of en- ergy ranging from 10.8 keVnr to 49.9 keVnr are measured under different drift field configurations. An electric field quenching on nuclear recoils in liquid argon is dis- covered and quantified for the first time. This quenching effect is also found to be drift field and recoil energy dependent. By varying the drift field amplitude from 100 V/cm to 1000 V/cm for each nuclear recoil energy, the quenching effect are measured as a function of nuclear recoil energy and drift field amplitude. Results from this measurement is used in the direct dark matter detection experiment to calculate the final sensitivity of direct dark matter search.

A separate work on the optimization of detector design for the XENON1T detector is also discussed in detail. Finite element simulation tool is used to design and optimize the electric field in XENON1T time projection chamber. As part of the design of XENON1T detector, electron transparency across metal grids of different geometrical configurations are also studied.

We reexamine the current direct dark matter (DM) detection data for several types of DM candidates, both assuming the Standard Halo Model (SHM) and in a halo-independent manner. We consider the potential signals for light WIMPs that have appeared in three direct detection searches: DAMA, CDMS-II-Si, and CoGeNT, and we analyze their compatibility with the null results of other direct detection experiments.

We first consider light WIMPs with exothermic scattering with nuclei (exoDM). Exothermic interactions favor light targets, thus reducing the importance of upper limits derived from Xe targets, the most restrictive of which is at present the LUX limit. In our SHM analysis the CDMS-II-Si and CoGeNT regions become allowed by these bounds, however the SuperCDMS limit rejects both regions for exoDM with isospin-conserving couplings. An isospin-violating coupling of the exoDM, in particular one with a neutron to proton coupling ratio of -0.8 (which we call "Ge-phobic"), maximally reduces the DM coupling to Ge and allows the CDMS-II-Si region to become compatible with all upper bounds. This is also clearly shown in our halo-independent analysis.

Next, we extend and correct a recently proposed maximum-likelihood halo-independent method to analyze unbinned direct DM detection data. Instead of the recoil energy as an independent variable, we use the minimum speed a DM particle must have to impart a given recoil energy to a nucleus. This has the advantage of allowing us to apply the method to any type of target composition and interaction, e.g. with general momentum and velocity dependence, and with elastic or inelastic scattering. We prove the method and provide a rigorous statistical interpretation of the results. As first applications, we find that for dark matter particles with elastic spin-independent interactions and neutron to proton coupling ratio $f_n/f_p=-0.7$ ("Xe-phobic", which reduces maximally the coupling to Xe), the WIMP interpretation of the signal observed by CDMS-II-Si is compatible with the constraints imposed by all other experiments with null results. We also find a similar compatibility for exothermic inelastic spin-independent interactions with $f_n/f_p=-0.8$.

Finally, we reexamine the interpretation of the annual modulation signal observed by the DAMA experiment as due to WIMPs with a spin-dependent coupling mostly to protons. We consider both axial-vector and pseudo-scalar couplings, and elastic as well as endothermic and exothermic inelastic scattering. We conclude that the DAMA signal is in strong tension with null results of other direct detection experiments, particularly PICASSO and KIMS.

We focus on two dark matter candidates: sterile neutrinos and primordial black holes (PBH). We explore the effects of non-standard pre-Big Bang Nucleosynthesis (pre-BBN) cosmologies, such scalar-tensor and kination cosmologies, on the abundance of sterile neutrinos over a large range of masses. In particular, sterile neutrinos of keV-scale mass represent a viable warm dark matter candidate whose decay can generate the putative 3.5 keV X-ray signal observed in galaxy and galaxy clusters. eV-scale sterile neutrinos can be the source of various accelerator/beam neutrino oscillation anomalies. Two production mechanisms are considered here, a collisional non-resonant Dodelson-Widrow (DW) mechanism and a resonant Shi-Fuller (SF) conversion (which requires a large lepton asymmetry). The DW mechanism is a freeze-in process, and the final abundance of sterile neutrinos using this production method is inversely proportional to the Hubble expansion rate. We find that in one of the scalar tensor models we consider, the sterile neutrino parameters necessary to generate the tentative 3.5 keV signal would be within reach of the TRISTAN upgrade to the ongoing KATRIN experiment as well as the planned upgrades to the HUNTER experiment, however the contribution to the dark matter density would be very small. In another scalar tensor model, sterile neutrinos could both generate the X-ray signal and comprise much of dark matter. In our study of resonant production, we find that the parameter space in which coherent and adiabatic resonant production can occur shifts with changing pre-BBN cosmology. We find that for a broad range of parameters (mass, mixing angle, lepton asymmetry), resonance can occur in the LSND/MiniBooNE and DANS/NEOSS experiments' preferred regions for at least one of the non-standard cosmologies we consider. With respect to PBH as dark matter candidates, we derive a new type of cosmology-independent bound. We consider the heating of the surrounding interstellar medium gas by dynamical friction and from the formation of accretion disks around intermediate mass $10-10^{5} M_\odot$ PBH. By estimating the cooling rate and assuming thermal equilibrium, we derive a new constraint. Light PBH with mass $10^{15}-10^{17}$ g emit significant Hawking radiation and are constrained by the same cooling argument. We extend this analysis to PBH with extreme spin, which results in stronger bounds compared to non-spinning PBH.

The channeling of the ion recoiling after a collision with a WIMP changes the ionization signal in direct detection experiments, producing a larger signal than otherwise expected. We give estimates of the fraction of channeled recoiling ions in NaI (Tl), Si, Ge, CsI, and solid Xe, Ar and Ne crystals using analytic models produced since the 1960's and 70's to describe channeling and blocking effects. We find that the channeling fraction of recoiling lattice nuclei is smaller than that of ions that are injected into the crystal and that it is strongly temperature dependent. Channeling is a directional effect which depends on the velocity distribution of WIMPs in the dark halo of our Galaxy and could lead to a daily modulation of the signal. We compute upper bounds to the expected amplitude of daily modulation due to channeling using our estimates of the channeling fractions. After developing the general formalism, we examine the possibility of finding a daily modulation due to channeling in the data already collected by the DAMA experiment. We find that even the largest daily modulation amplitudes would not be observable for WIMPs in the standard halo in the 13 years of data taken by the DAMA collaboration. For these to be observable the DAMA total rate should be 1/40 of what it is or the total DAMA exposure should be 40 times larger. The daily modulation due to channeling will be difficult to measure in future experiments.

We study two topics in this thesis: the halo-independent (HI) analysis method for dark matter (DM)-electron scattering and the phenomenology of sterile neutrinos ($\nu_s$s) produced in the evaporation of early Universe black holesWe demonstrate how the HI analysis can be applied to sub-GeV DM scattering off electrons for noble gas targets such as Xe and semiconductor targets such as Ge and Si. In the HI analysis method, properties of the local DM halo velocity distribution are inferred from direct DM detection data, which allows the comparison of different data sets without making any assumption on the uncertain local dark halo characteristics. This method had previously been developed for and applied only to DM scattering off nuclei. We additionally show that in-medium effects could significantly affect HI analysis results for semiconductor targets Ge and Si and thus are essential for proper inference of local DM halo characteristics from direct DM detection data. For $\nu_s$s as DM candidates, we discuss in detail the characteristics and phenomenology of $\nu_s$s that minimally couple only to active neutrinos and are produced in the evaporation of early Universe primordial black holes (PBHs), a process we call ``PBH sterile neutrinogenesis". Contrary to previously studied $\nu_s$ production mechanisms, this novel mechanism does not depend on the active-sterile mixing. The resulting $\nu_s$s have a distinctive spectrum and are produced with larger energies than in typical scenarios. This characteristic enables $\nu_s$s to be warm DM in the unusual $0.3$ MeV to $0.3$ TeV mass range if PBHs do not matter-dominate the Universe before evaporating. When PBHs matter-dominate before evaporating, the possible coincidence of induced gravitational waves associated with PBH evaporation and astrophysical X-ray observations from $\nu_s$ decays would constitute a distinct signature of our scenario.

A vast number of independent astrophysical and cosmological observations suggest that the dominant form of matter in the Universe, known as dark matter, is neither luminous nor baryonic. Despite nearly half a decade of research, the non-gravitational nature of dark matter, if any, remains a mystery. Motivated primarily by preferred theoretical extensions of the Standard Model and a relatively simple production mechanism, the weakly interacting massive particle (WIMP) has long been considered to be among the most appealing dark matter particle candidates. This dissertation is comprised of largely independent works that focus on understanding and constraining various signals that could arise from WIMP dark matter. Specifically, Chapters 2 and 3 address the impact that non-standard astrophysics and particle physics could have on the observed scattering rate in direct dark matter detection experiments; Chapter 4 presents a halo-dependent and an halo-independent update on the viability of a dark matter interpretation of the CDMS-II-Si data; Chapter 5 generalizes the halo-independent analysis formalism such that the compatibility of multiple experiments can be assessed, and the preferred halo-independent parameter space can be identified, for global likelihoods comprised of at least one extended likelihood; Chapter 6 discusses the prospects for detecting gamma-rays from dark matter annihilating in local dark matter subahlos; Chapter 7 presents updated constraints on simplified dark matter models that are consistent with the Galactic Center excess; and Chapter 8 discusses the extent to which future direct detection experiments may be able to elucidate the high-energy dark matter theory from observations of low-energy nuclear recoils.

Many dark matter candidates, including asymmetric Weakly Interacting Massive Particles (WIMPs) and sterile neutrinos, are produced in the very early Universe, prior to Big Bang Nucleosynthesis (BBN). We show that the relic abundance of asymmetric WIMPs and sterile neutrinos can be very sensitive to the expansion rate of the Universe prior to BBN. In particular, we find that if the production of asymmetric WIMPs occurs during a non-standard cosmological phase, a larger WIMP annihilation cross section is required to produce the present dark matter density than if the WIMPs were produced during a standard, radiation dominated phase. Because of this, the present dark matter annihilation rate could be larger than that of symmetric dark matter produced in the standard cosmology. We also show that if the production of sterile neutrinos occurs during a non-standard cosmological phase, the relic number density of sterile neutrinos could be reduced with respect to the number expected in the standard cosmology, consequently relaxing current bounds on active-sterile neutrino mixing. Finally, we examine whether low reheating temperature cosmologies are allowed by current Cosmic Microwave Background measurements. We find the allowed range of reheating temperatures using monomial and binomial inflationary potentials, and a variety of reheating models. We show that an inflationary model with a φ^1 potential and canonical reheating allows the possibility that dark matter could be produced during the reheating epoch, instead of when the Universe is radiation dominated.