Browsing by Author "Sirkiä, Topi"

The QCD axion arises as a necessary consequence of the popular Peccei-Quinn solution to the strong CP problem in particle physics. The axion turns out to very naturally possesses all the usual qualities of a good dark matter (DM) candidate. Having the potential to solve two major problems in particle cosmology in one fell swoop makes the axion a very attractive prospect. In recent years, the weakening of the traditional WIMP dark matter paradigm and axion search experiments just beginning to reach the sensitivities required to look for the QCD axion have further increased interest in axion physics. In this thesis, the basics of axion physics are reviewed, and an in-depth exposition of common direct detection experiments and astrophysical and laboratory limits is given. Particular emphasis is placed on direct detection by using the axion-photon coupling as it is the only coupling in which experimental sensitivity is enough to probe the QCD axion. The benchmark experiments of light-shining-through-wall (LSTW), helioscopes and cavity haloscopes are given a thorough theoretical treatment. Other couplings and related experiments are relevant when looking for axion-like particles (ALPs), which are postulated by various extensions of the Standard Model but which do not solve the strong CP problem. A general overview of the prevalent ALP-searches is given. Most of the described experimental setups, with some exceptions, are actually searches for very general weakly interacting particles, WISPs, with a certain coupling. The searches are thus well motivated regardless of the future standing of the QCD axion. A chapter is dedicated to axion dark matter and its creation mechanisms, in particular the misalignment mechanism. Two scenarios are mapped out, depending on whether the Peccei-Quinn symmetry spontaneously breaks before or after inflation. Both cases have experimental implications, which are compared. These considerations motivate an axion dark matter window which should be prioritized by experiments. A significant part of this thesis is dedicated to mapping out the experimental landscape of axions today. The up-to-date astrophysical and laboratory limits on the most prominent axion couplings along with projections of some near-future experiments are compiled into a set of exclusion plots.