Abstract
We show that dark matter can be accounted for by an axion that solves the strong CP problem, but is much lighter than usual due to a Z \U0001d4a9 symmetry.The whole mass range from the canonical QCD axion down to the ultra-light regime is allowed, with 3 ≤ \U0001d4a9 ≲ 65. This includes the first proposal of a "fuzzy dark matter" QCD axion with m a ∼ 10-22 eV.A novel misalignment mechanism occurs — trapped misalignment — due to the peculiar temperature dependence of the Z \U0001d4a9 axion potential. The dark matter relic density is enhanced because the axion field undergoes two stages of oscillations: it is first trapped in the wrong minimum, which effectively delays the onset of true oscillations. Trapped misalignment is more general than the setup discussed here, and may hold whenever an extra source of Peccei-Quinn breaking appears at high temperatures. Furthermore, it will be shown that trapped misalignment can dynamically source the recently proposed kinetic misalignment mechanism. All the parameter space is withintantalizing reach of the experimental projects for the next decades. For instance, even Phase I ofCASPEr-Electric could discover this axion.
Highlights
Were an axion-like particle (ALP) to be ever discovered, it would be compelling to explore whether it has something to do with the strong CP problem [1,2,3,4], and/or whether it can be a viable dark matter (DM) candidate [5,6,7]
We showed here that the evolution of the axion field through the cosmological history departs drastically from both the standard one and from previously considered mirror world scenarios
We identified a novel axion production mechanism which holds whenever fa 3.2 × 1017 GeV: trapped misalignment, which is a direct consequence of the temperature dependence of the axion potential
Summary
Were an axion-like particle (ALP) to be ever discovered, it would be compelling to explore whether it has something to do with the strong CP problem [1,2,3,4], and/or whether it can be a viable dark matter (DM) candidate [5,6,7]. While an early stage of oscillations has been previously proposed in the literature in order to adiabatically suppress the axionic energy density and/or to solve the isocurvature problem [54,55,56,57], our trapped misalingment differs from previous works in that the modification of the potential (from the high temperature large axion mass to the exponentially suppressed zero temperature mass) is strongly nonadiabatic and, when active, always leads to an enhancement of the relic density. It will be shown that, in some regions of the parameter space, trapped misalignment will automatically source the recently proposed kinetic misalignment mechanism [61] In the latter, a sizeable initial axion velocity is the source of the axion relic abundance as opposed to the conventionally assumed initial misalignment angle. The structure of the paper can be inferred from the Table of Contents
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