Probing the Phenomenology of Dark Matter from Decoupled Freeze-Out

TL;DR

This paper investigates a dark matter model with a mixed scalar-pseudoscalar mediator, focusing on decoupled freeze-out, and analyzes its relic density and experimental constraints, revealing viable parameter space for future exploration.

Contribution

It introduces a decoupled freeze-out mechanism with a mixed mediator, providing new insights into dark matter relic density and experimental bounds.

Findings

Decoupled freeze-out can produce the correct relic density.

Viable parameter space remains after applying experimental constraints.

The model evades standard CMB limits while allowing indirect detection signals.

Abstract

We consider a model of dark matter where the mediator corresponds to a superposition of a scalar and pseudoscalar, and the scenario where, after reheating, the number densities of the dark sector particles, i.e. the dark matter and the mediators, are negligible. If the coupling of the mediators to the Standard Model is feeble, but the coupling to the dark matter is large enough, the dark sector may reach equilibrium at a temperature distinct from that of the thermal bath. The relic density is then said to be obtained via decoupled freeze out (DFO). We focus on the $s$-wave annihilation scenario, which particularly benefits from the DFO mechanism by evading standard CMB limits while still yielding indirect detection signals. We calculate the relic density by solving a set of four coupled Boltzmann equations for the number densities of the dark sector particles and the energy transfer from the light to dark sector. We finally perform a thorough analysis of experimental bounds on this scenario, namely from indirect detection and the CMB, as well as from BBN, and find that, while there are considerable constraints on the parameter space where the correct relic density is obtained, a viable region remains to be explored.