Probing Light Thermal Dark-Matter With a Higgs Portal Mediator

TL;DR

This paper investigates light dark matter models with a Higgs portal mediator, identifying parameter space regions constrained by current experiments and highlighting future prospects for detection.

Contribution

It provides a systematic analysis of light DM with Higgs portal mediators, clarifying which scenarios are viable or excluded by existing and upcoming experiments.

Findings

Mediator heavier than DM is ruled out by collider and detection limits.

Lighter mediator with DM annihilating to visible mediators remains viable.

Future experiments will significantly improve constraints on these models.

Abstract

We systematically study light (< few GeV) Dark Matter (DM) models that thermalize with visible matter through the Higgs portal and identify the remaining gaps in the viable parameter space. Such models require a comparably light scalar mediator that mixes with the Higgs to avoid DM overproduction and can be classified according to whether this mediator decays (in)visibly. In a representative benchmark model with Dirac fermion DM, we find that, even with conservative assumptions about the DM-mediator coupling and mass ratio, the regime in which the mediator is heavier than the DM is fully ruled out by a combination of collider, rare meson decay, and direct detection limits; future and planned experiments including NA62 can further improve sensitivity to scenarios in which the Higgs portal interaction does not determine the DM abundance. The opposite, regime in which the mediator is lighter than the DM and the latter annihilates to pairs of visibly-decaying mediators is still viable, but much of the parameter space is covered by rare meson decay, supernova cooling, beam dump, and direct detection constraints. Nearly all of these conclusions apply broadly to the simplest variations (e.g. scalar or asymmetric DM). Future experiments including SHiP, NEWS, and Super-CDMS SNOLAB can greatly improve coverage to this class of models.