The domain of thermal dark matter candidates

TL;DR

This paper explores the parameter space of thermal dark matter candidates, considering different decoupling scenarios, observational constraints, and the impact of a hidden sector temperature, providing a comprehensive framework for understanding viable models.

Contribution

It introduces a general parameter space framework for thermal dark matter, incorporating hidden sector temperature differences and observational bounds, with three illustrative simplified scenarios.

Findings

Boundaries set by relativistic and unitarity decoupling limits.

Constraints from big bang nucleosynthesis and free streaming.

Demonstration of scenarios fitting into the parameter space.

Abstract

We consider, in general terms, the possible parameter space of thermal dark matter candidates. We assume that the dark matter particle is fundamental and was in thermal equilibrium in a hidden sector with a temperature $T'$, which may differ from that of the Standard Model temperature, $T$. The candidates lie in a region in the $T'/T$ vs. $m_{\rm dm}$ plane, which is bounded by both model-independent theoretical considerations and observational constraints. The former consists of limits from dark matter candidates that decoupled when relativistic (the relativistic floor) and from those that decoupled when non-relativistic with the largest annihilation cross section allowed by unitarity (the unitarity wall), while the latter concerns big bang nucleosynthesis ($N_{\rm eff}$ ceiling) and free streaming. We present three simplified dark matter scenarios, demonstrating concretely how each fits into the domain.