Bound states of WIMP dark matter in Higgs-portal models. Part II. Thermal decoupling

TL;DR

This paper investigates how Higgs-mediated interactions and bound-state formation affect the thermal decoupling of multi-TeV WIMP dark matter, significantly impacting relic density calculations and dark matter properties.

Contribution

It provides the first detailed calculation of Higgs-induced bound-state effects on WIMP relic density, revealing their importance in dark matter phenomenology.

Findings

Bound-state formation via Higgs emission reduces relic density.

Dark matter may be heavier or less coupled than previously thought.

Effects are more significant for larger multiplets.

Abstract

The Higgs doublet can mediate a long-range interaction between multi-TeV particles coupled to the Weak interactions of the Standard Model, while its emission can lead to very rapid bound-state formation processes and bound-to-bound transitions. Using the rates calculated in a companion paper, here we compute the thermal decoupling of multi-TeV WIMP dark matter coupled to the Higgs, and show that the formation of metastable dark matter bound states via Higgs-doublet emission and their decay decrease the relic density very significantly. This in turn implies that WIMP dark matter may be much heavier than previously anticipated, or conversely that for a given mass, the dark matter couplings to the Higgs may be much lower than previously predicted, thereby altering the dark matter phenomenology. While we focus on a minimal singlet-doublet model in the coannihilation regime, our calculations can be extended to larger multiplets where the effects under consideration are expected to be even more significant.