Direct and Indirect Searches for Axion Dark Matter

Abstract

The majority of the matter in the Universe is non-luminous and unaccounted for by any known particle, making the unknown nature of dark matter one of the most urgent problems in fundamental physics. Amidst a broad landscape of particles proposed to explain the dark matter, axions have emerged as a particularly well-motivated candidate as they naturally arise in extensions of the Standard Model and can simultaneously reproduce the observed dark matter abundance while solving other outstanding mysteries in particle physics. Despite this, axions have remained largely unprobed, and new insights and innovative approaches are required to carefully test the axion dark matter hypothesis. This thesis aims to advance prospects for axion detection by identifying how axion signals may appear, developing optimized searches for these signals, and implementing robust analysis strategies. I will begin by showing how simulations of axion production in the early universe can direct search efforts toward the best-motivated mass range for axions that solve the Strong textit{CP} Problem related to the absence of a neutron electric dipole moment in quantum chromodynamics. I will then discuss the development of rigorous analysis frameworks for axion direct detection and their application to the search for axion dark matter with the ABRACADABRA detector. Lastly, I will show how astrophysical observations with textit{X}-ray and radio telescopes can be used in novel searches for axion dark matter. This thesis contributes to an increasingly comprehensive search program that will either discover or exclude axion dark matter in the coming years.