Heating up Peccei-Quinn scale

TL;DR

This paper proposes a novel scenario for QCD axion dark matter production involving a time-varying Peccei-Quinn scale, which allows axion abundance generation independent of mass and is constrained by cosmological and astrophysical data.

Contribution

It introduces a new mechanism where the Peccei-Quinn scale decreases early on, enabling axion production via parametric resonance unaffected by universe expansion.

Findings

Axion dark matter can be produced efficiently during parametric resonance decay.

The scenario's viability depends on the ratio of radial field to axion particle densities.

Constraints narrow the Peccei-Quinn scale to around 10^8 GeV, testable by future axion searches.

Abstract

We discuss production of QCD axion dark matter in a novel scenario, which assumes time-varying scale of Peccei-Quinn symmetry breaking. The latter decreases as the Universe's temperature at early times and eventually stabilises at a large constant value. Such behavior is caused by the portal interaction between the complex field carrying Peccei-Quinn charge and a Higgs-like scalar, which is in thermal equilibrium with primordial plasma. In this scenario, axions are efficiently produced during the parametric resonance decay of the complex Peccei-Quinn field, relaxing to the minimum of its potential in the radiation-dominated stage. Notably, this process is not affected by the Universe's expansion rate and allows to generate the required abundance of dark matter independently of an axion mass. Phenomenological constraints on the model parameter space depend on the number density of radial field fluctuations, which are also generically excited along with axions, and the rate of their thermalization in the primordial plasma. For the ratio of radial field and axion particles number densities larger than $\sim 0.01$ at the end of parametric resonance decay, the combination of cosmological and astrophysical observations with the CAST limit confines the Peccei-Quinn scale to a narrow range of values $\sim 10^{8}~\mbox{GeV}$, - this paves the way for ruling out our scenario with the near future searches for axions.