Bound states of WIMP dark matter in Higgs-portal models. Part I. Cross-sections and transition rates

TL;DR

This paper explores how the Higgs doublet influences the formation and decay of bound states of multi-TeV WIMP dark matter, significantly affecting relic density calculations in Higgs-portal models.

Contribution

It provides the first detailed calculation of Higgs-mediated bound state formation and transition rates for fermionic dark matter in the unbroken electroweak phase.

Findings

Higgs doublet mediates long-range forces affecting dark matter annihilation.

Higgs emission enables rapid bound-state formation and transitions.

Bound state effects can substantially reduce dark matter relic density.

Abstract

We investigate the role of the Higgs \emph{doublet} in the thermal decoupling of multi-TeV dark matter coupled to the Weak interactions of the Standard Model and the Higgs. The Higgs doublet can mediate a long-range force that affects the annihilation processes and binds dark matter into bound states. More importantly, the emission of a Higgs doublet by a pair of dark matter particles can give rise to extremely rapid monopole bound-state formation processes and bound-to-bound transitions. We compute these effects in the unbroken electroweak phase. To this end, we consider the simplest renormalisable fermionic model, consisting of a singlet and a doublet under $SU_{L}(2)$ that are stabilised by a $\mathbb{Z}_2$ symmetry, in the regime where the two multiplets coannihilate. In a companion paper, we use the results to show that the formation of metastable bound states via Higgs-doublet emission and their decay decrease the relic density very significantly.