Non-equilibrium dynamics of Axion-like particles: the quantum master equation

Understand

We study the non-equilibrium dynamics of Axion-like particles (ALP) coupled to Standard Model degrees of freedom in thermal equilibrium.

• The Quantum Master Equation (QME) for the (ALP) reduced density matrix is derived to leading order in the coupling of the (ALP) to the thermal bath, but to \emph{all} orders of the bath couplings to degrees of freedom within or beyond the Standard Model other than the (ALP).
• The (QME) describes the damped oscillation dynamics of an initial misaligned (ALP) condensate, thermalization with the bath, decoherence and entropy production within a unifying framework.
• The (ALP) energy density $\mathcal{E}(t)$ features two components: a ``cold'' component from the misaligned condensate and a ``hot'' component from thermalization with the bath, with $\mathcal{E}(t)= \mathcal{E}_{c}\,e^{-\gamma(T)\,t}+\mathcal{E}_{h}(1-e^{-\gamma(T)\,t})$ thus providing a ``mixed dark matter'' scenario.