Axion-Like-Particle Dark Matter Beyond the Standard Paradigm

Abstract

The nature of dark matter remains one of the most important questions in modern physics. Historically, the most investigated dark matter candidates have been thermal in origin. Such candidates are championed by the paradigm of weakly interacting massive particles (WIMPs). However, with the continued non-discovery of WIMPs by the many dedicated direct detection searches as well as the non-observation of supersymmetry and other beyond-Standard-Model theories related to the WIMP paradigm, we are motivated to look elsewhere and explore new scenarios.

An interesting possibility is that dark matter may not be of a thermal origin. Non-thermal production mechanisms of dark matter suggest that dark matter may have a coupling to the Standard Model many orders of magnitude smaller than the range suggested by WIMPs. This allows for the possibility of dark matter being both very light and cosmologically stable. The canonical examples of such light, non-thermally produced dark matter candidates are the axion-like-particles, which in recent years have seen a resurgence of attention from theorists and experimentalists alike.

Axion-like-particles are pseudoscalar particles which are natural ultra-light dark matter candidates because even minimal models have an inherent production mechanism known as the misalignment mechanism. This mechanism relies on the observation that an oscillating scalar field behaves as cold dark matter and that the most generic initial conditions inevitably lead to such oscillations. However, the energy density predicted by the misalignment mechanism strongly depends on the initial condition of the axion field. In much of the parameter space that is experimentally accessible, the standard misalignment mechanism underproduces dark matter. The key motivation for this thesis is to question the standard assumption on the initial conditions for the axion field.

This thesis is devoted to the investigation of novel production mechanisms of axion-like-particle dark matter, which go beyond the standard paradigm by exploring initial conditions involving non-zero kinetic energy of the axion-like-particle field. This family of scenarios goes under the common name of kinetic misalignment. Our exploration of kinetic misalignment is divided into two stages: In the first stage, part II of this thesis, we investigate the dynamics of such scenarios independently of how the initial conditions are realized. In particular, we study the role of parametric resonance and the phenomenon of axion fragmentation. We find that this form of parametric resonance is indeed efficient in much of the relevant parameter space and that the produced relic, therefore, has a non-trivial momentum spectrum which could lead to observational prospects in the form of dark matter mini-clusters. In the second stage, part III of this thesis, we investigate how the assumed initial conditions can be implemented in scenarios inspired by Affleck-Dine baryogenesis. This allows us to present the full story of how and under which constraints the standard paradigm can be extended to such scenarios of kinetic misalignment. We find that a KSVZ-like model with a nearly-quadratic potential can support much of the interesting parameter space in the 10^{-6} eV < m_a < 10 eV regime if the spectrum of primordial fluctuations can be adequately suppressed. In this case, a period of kination can lead to an associated signature in amplified primordial gravitational waves.

In summary, we show that axion-like-particle dark matter can be motivated in nearly all of the [m_a,f_a] parameter space that is currently unconstrained by experiments. In the coming years, a large number of searches will extend the experimental reach. These experiments have the prospect of not only discovering a QCD-axion or an axion-like-particle but also having that discovery constitute all of the observed dark matter.