Heating up Peccei-Quinn scale

Abstract

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 ∼ 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 ∼ 108 GeV, — this paves the way for ruling out our scenario with the near future searches for axions.