Roadmap to Thermal Dark Matter Beyond the WIMP Unitarity Bound

TL;DR

This paper provides a comprehensive analysis of thermal relic freezeout processes, deriving generalized relations for dark matter abundance and mass bounds, extending beyond traditional WIMP models to include higher mass scenarios and metastable states.

Contribution

It introduces a unified analytic framework for arbitrary freezeout processes, generalizing the dark matter mass-coupling relationship and establishing an approximate unitarity bound.

Findings

Relic abundance estimates for arbitrary freezeout processes.

Dark matter can have masses up to the Planck scale.

Existence of nearly degenerate states and metastability at high masses.

Abstract

We study the general properties of the freezeout of a thermal relic. We give analytic estimates of the relic abundance for an arbitrary freezeout process, showing when instantaneous freezeout is appropriate and how it can be corrected when freezeout is slow. This is used to generalize the relationship between the dark mater mass and coupling that matches the observed abundance. The result encompasses well-studied particular examples, such as WIMPs, SIMPs, coannihilation, coscattering, inverse decays, and forbidden channels, and generalizes beyond them. In turn, this gives an approximate perturbative unitarity bound on the dark matter mass for an arbitrary thermal freezeout process. We show that going beyond the maximal masses allowed for freezeout via dark matter self-annihilations (WIMP-like, $m_{\rm DM}\gg\mathcal{O}(100~\rm TeV)$) predicts that there are nearly degenerate states with the dark matter and that the dark matter is generically metastable. We show how freezeout of a thermal relic may allow for dark matter masses up to the Planck scale.