Axion dark matter from frictional misalignment

TL;DR

This paper investigates how thermal friction from sphaleron processes affects axion-like particle dark matter abundance, providing analytical tools and exploring parameter space modifications that align with observed dark matter levels.

Contribution

It introduces an analytical formula for the frictional adiabatic invariant and explores how thermal friction alters axion relic density across different scenarios.

Findings

Thermal friction can significantly modify axion relic abundance.

The derived formula enables accurate relic density calculations in complex scenarios.

Parameter space for axion dark matter can be expanded to match observed relic density.

Abstract

We study the impact of sphaleron-induced thermal friction on the axion dark-matter abundance due to the interaction of an axion-like particle (ALP) with a dark non-abelian gauge sector in a secluded thermal bath. Thermal friction can either enhance the axion relic density by delaying the onset of oscillations or suppress it by damping them. We derive an analytical formula for the \emph{frictional adiabatic invariant}, which remains constant along the axion evolution and which allows us to compute the axion relic density in a general set-up. Even in the most minimal scenario, in which a single gauge group is responsible for both the generation of the ALP mass and the friction force, we find that the resulting dark-matter abundance from the misalignment mechanism deviates from the standard scenario for axion masses $m_a\gtrsim 100 \; {\rm eV}$. We also generalize our analysis to the case where the gauge field that induces friction and the gauge sector responsible for the ALP mass are distinct and their couplings to the axion have a large hierarchy as can be justified by means of alignment or clockwork scenarios. We find that it is easy to open up the ALP parameter space where the resulting axion abundance matches the observed dark-matter relic density both in the traditionally over- and underabundant regimes. This conclusion also holds for the QCD axion.