Ariel Zhitnitsky

Over two decades of development since its establishment, the axion quark nugget (AQN) model is one of the best-studied macroscopic dark matter (DM) candidates, with a mass of grams and a size of around 0.1 micrometres. It naturally explains the observed similarity between the dark and visible densities in the Universe, i.e. Ωᴅᴍ~Ω?i?, with no fitting parameters, while many conventional DM candidates such as the weakly interacting massive particles (WIMPs) and axions do not. This dissertation presents numerous key developments of the AQN model, including the formation mechanism for AQNs in the early Universe and new search strategies such as axion detection and a global network of synchronized detectors. Lastly, we discuss potential evidence of AQNs from the anomalous events recently observed by the Telescope Array (TA) and ANtarctic Impulse Transient Antenna (ANITA) experiments.

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The axion quark nugget (AQN) model was initially proposed with the motivation to explain the observed similarity between the visible and dark matter abundances in the Universe. AQNs are dense objects made of standard model quarks in color superconducting (CS) phase. AQNs can be made of matter as well as antimatter. Matter AQNs and antimatter AQNs together form the dark matter, while the disparity between them will lead to the observed matter-antimatter asymmetry. Thus, the similarity between visible and dark matter abundances can be naturally explained since they have the same origin in the AQN framework.This thesis focuses on recent developments in model building and some potential observational evidence of AQNs. First, we show how the coherent nonzero axion field in the early Universe generates the disparity between matter and antimatter AQNs. Then, we calculate the real-time evolution of an AQN from its initial state as a closed axion domain wall with baryon charge trapped inside to its final CS state. Next, we show that for the most part of axion parameter space, AQNs are the dominant part of dark matter compared to the contribution of the free axions from the misalignment mechanism. After that, we calculate the size distribution of AQNs based on percolation theory. We also demonstrate that after formation, the size distribution can survive the subsequent evolution in the early Universe. Finally, we study potential observational evidence of the AQN model, focusing on the following two phenomena: the impulsive radio events in quiet solar corona recorded by the Murchison Widefield Array and the seasonal variation of the near-Earth X-ray background observed by the XMM-Newton observatory.

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We use a deformed “center-stablised” gauge theory, which can be brought into a weak coupling regime while remaining confined and gapped, as a toy model to study some ideas from real QCD. The deformed model has the correct nontrivial θ-dependence and degeneracy of topological sectors conjectured for QCD, and is, apparently, smoothly connected to the strongly coupled undeformed Yang-Mills, so that we can perhaps expect to get some qualitative insights into QCD. We demonstrate the presence of a nondispersive contact term in the topological susceptibility, which contributes with the opposite sign to normal dispersive contributions coming from physical propagating degrees of freedom. We further show that, despite the system being completely gapped with no massless physical degrees of freedom, the system has a Casimir-like, power scaling, dependence on boundaries, in contrast with the naive expectation that a system with only massive degrees of freedom should have a weak (exponentially small) dependence on long distance effects. This behaviour suggests the possibility for a solution for the cosmological dark energy problem coming from the strongly coupled QCD sector on a manifold with a boundary, which would have the correct sign and be of the correct order of magnitude. Next, we investigate the interaction between point-like topological charges (monopoles) and extended sheet-like topological defects (domain walls) in attempt to explain some recent lattice QCD results suggesting that extended topological objects are more important to understanding the relevant field configurations in QCD than the instantons traditionally expected. Finally, we derive the existence of excited metastable vacuum states and calculate their decay rate to the true ground state of the theory, comparing with the expected results discussed years ago in proper QCD. The presence of metastable vacuum states with a nonzero effective θ parameter, like those present in the deformed model, could explain P and CP violation in heavy ion collisions observed on an event by event basis, which seem to average away over many events.

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I present a dark matter model in which the dark matter is composed of very heavy “nuggets” of Standard Model quarks and antiquarks. This model was originally motivated by the fact that the matter and dark matter mass densities are observed to have similar scales. If these two forms of matter originate through completely distinct physical processes then their densities could easily have existed at vastly different scales. However, if the dark and the visible matter are co-produced, this similarity in scales is a natural outcome. In the model considered here dark matter and the baryonic matter share an origin in Standard Model strong force physics. The main goal of this work is to establish the testable predictions of this model. The physical properties of the nuggets are set by well understood nuclear physics and quantum electrodynamics, allowing many observable consequences to be predicted. To this end, I devote special attention to the structure of the surface layer of the nuggets from which the majority of observable consequences arise. With this basic picture of nugget structure in place, I will discuss the consequences of their interactions with a number of different environments. Particular attention is given to the galactic centre and to the early universe, as both are sufficiently dense to allow for significant levels of matter-dark matter interaction. The emitted radiation, in both cases, is shown to be consistent with observations. Finally, I discuss the consequences of a nugget striking the earth. In this context, I will demonstrate that the nuggets produce effects observable in cosmic ray detectors. Based on these considerations, I discuss the nugget detection potential for experiments primarily devoted to the study of high energy cosmic rays.

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This thesis introduces the idea of a topological current that flows in regions with large magnetic fields, dense matter, and parity violation. We propose that such a current exists in the cores of neutron stars and may be responsible for the large proper motion (kicks) observed in some pulsars. This current is similar to the charge separation effect and chiral magnetic effect that may be responsible for parity (℘) and charge conjugation-parity (C℘) violation observed at the Relativistic Heavy Ion Collider (RHIC). We start by deriving the topological current two ways. The first is a macroscopic derivation where we appeal to an anomaly induced by the presence of a fictitious axial field. The second method is microscopic, in which we consider how the modes of the Dirac equation in a magnetic field and chemical potential contribute to the current. We then discuss in great detail the elements necessary for a topological current to exist in a dense star.Our concern then rests with calculating the magnitude of topological currents in the many phases of matter thought to exist in dense stars. We choose four representative processes to investigate: nuclear matter, hyperons, kaon condensates, and strange quark matter. We then suppose that this current may somehow transfer its momentum out of the star, either by being physically ejected or by emitting radiation, causing a kick. We also discuss how the current may induce magnetic helicity and a toroidal magnetic field in the core of the star.We end by discussing the topological current in terms of the AdS/CFT correspondence, a powerful tool that allows one to obtain results from strongly coupled field theories by transferring the problem to the language of a weakly coupled gravitational theory. We introduce a toy model to how one might introduce topological currents into the AdS/CFT framework.

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