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The axion is a hypothetical elementary particle postulated by the Peccei-Quinn theory to resolve the strong CP problem in QCD. If axions exist and have low mass, they are a candidate for dark matter as well. So far our knowledge of the…
First I will review the QCD theta problem and the Peccei-Quinn solution, with its new particle, the axion. I will review the possibility of the axion as dark matter. If PQ symmetry was restored at some point in the hot early Universe, it…
We discuss phenomenological criteria for defining "axion windows", namely regions in the parameter space of the axion-photon coupling where realistic models live. Currently, the boundaries of this region depend on somewhat arbitrary…
We analyze the mass of the axino, the fermionic superpartner of the axion, in general supergravity models incorporating a Peccei--Quinn--symmetry and determine the cosmological constraints on this mass. In particular, we derive a simple…
We analyze the strong CP problem and the implications for axion physics in the context of $U_1$ vector leptoquark models, recently put forward as an elegant solution to the hints of lepton flavor universality violation in B-meson decays. It…
The QCD axion needs not be an exact pseudoscalar for solving the strong CP problem. Its imperfectness can play a profound role cosmologically. We propose effective operators, where the Peccei-Quinn field linearly couples to Standard Model…
We study the parameter space of cold dark matter axions in two cosmological scenarios with non-standard thermal histories before Big Bang nucleosynthesis: the Low Temperature Reheating (LTR) cosmology and the kination cosmology. If the…
The axino is a promising candidate for dark matter in the Universe. It is electrically and color neutral, very weakly interacting, and could be - as assumed in this study - the lightest supersymmetric particle, which is stable for unbroken…
The Standard Model of particle physics can be extended to include sterile (right-handed) neutrinos or axions to solve the dark matter problem. Depending upon the mixing angle between active and sterile neutrinos, the latter have the…
Dark matter constitutes about $23\%$ of the total energy density of the universe but its properties are still little known besides that it should be composed by cold and weakly interacting particles. Many beyond standard model theories can…
The axion not only solves the strong CP puzzle, but it also may be the main constituent of cold dark matter. We review the axion dark matter predictions for the case that the Peccei-Quinn symmetry is restored after inflation.
Axion dark matter in high-scale inflation is subject to the isocurvature constraint, since quantum fluctuations of the axion field during inflation may exceed the current CMB bound. One conventional way to suppress these fluctuations is to…
We investigate the axion dark matter scenario (ADM), in which axions account for all of the dark matter in the Universe, in light of the most recent cosmological data. In particular, we use the Planck temperature data, complemented by WMAP…
We examine the minimal supergravity (mSUGRA) model under the assumption that the strong CP problem is solved by the Peccei-Quinn mechanism. In this case, the relic dark matter (DM) abundance consists of three components: {\it i}). cold…
Relativistic axions are good candidates for the dark radiation for which there are mounting observational hints. The primordial decays of heavy fields produce axions which are ultra-energetic compared to thermalised matter and inelastic…
We study in detail a supersymmetric Peccei-Quinn model, which has a DFSZ and a KSVZ version. The fields breaking the Peccei-Quinn (PQ) symmetry correspond to flat directions (flaton fields) and have unsuppressed couplings when PQ symmetry…
Axions currently provide the most compelling solution to the strong CP problem. These particles may be copiously produced in the early universe, including via thermal processes. Therefore, relic axions constitute a hot dark matter component…
We reanalyze the conditions under which we have a primordial thermal population of axions. Compared with previous studies, we find other processes, involving gluons and quarks, that dominate at high temperatures. We conclude that if the…
The QCD axion is a good dark matter candidate. The observed dark matter abundance can arise from misalignment or defect mechanisms, which generically require an axion decay constant $f_a \sim \mathcal{O}(10^{11})$ GeV (or higher). We…
We consider supersymmetric (SUSY) models wherein the strong CP problem is solved by the Peccei-Quinn (PQ) mechanism with a concommitant axion/axino supermultiplet. We examine R-parity conserving models where the neutralino is the lightest…