Dark Matter Coherent Freeze-Out Mechanism: A Comprehensive Analysis

Abstract

Both Weakly-Interacting Massive Particle (WIMP) and Axion-Like Particle (ALP) are attractive particle dark matter candidates. In this paper we consider a scenario in which both WIMP and ALP are present. The two species interact via a non-derivative quadratic coupling modeled on the axion-nucleon interaction in QCD. We show that even a small (Planck-suppressed) coupling can drastically modify the cosmological history of both species. In the very early Universe, the ALP field acquires a vacuum expectation value, and the system undergoes an inverse phase transition at a temperature around or below the weak scale. WIMPs may remain in equilibrium far beyond the usual freeze-out time, eventually decoupling via a novel "coherent freeze-out" mechanism. WIMP annihilation cross sections several orders of magnitude above the canonical thermal target value can be consistent with the observed relic density in the coherent freeze-out scenario. At the same time, coherent oscillations of the ALP field do not begin until after the phase transition, and the initial conditions for these oscillations are set by the thermal physics of the phase transition. The ALP relic density is then predicted unambiguously in terms of the WIMP mass and coupling. We term this scenario "ALP miracle", since the observed relic density is obtained when these parameters are around their natural values, the weak and the Planck scale. The ALP miracle scenario prefers the ALP mass above about $10^{-3}\,\mathrm{eV}$, far larger than other popular ALP dark matter models. The ALP miracle parameter space overlaps with a currently allowed region of the QCD axion band, a large fraction of which is expected to be probed by the proposed IAXO experiment.