Freeze-out Forbidden Dark Matter in the Hidden Sector in the Mass Range from sub-GeV to TeV

TL;DR

This paper explores a forbidden freeze-out mechanism for scalar-mediated dark matter in a hidden sector, analyzing its parameter space and potential experimental signatures across a wide mass range from sub-GeV to TeV.

Contribution

It introduces a simple secluded vector dark matter model with forbidden channels, detailing its relic abundance, parameter constraints, and experimental testability in the scalar portal framework.

Findings

Allowed mass splitting varies with dark matter mass

Sub-GeV region permits larger mass splittings

TeV region requires mass splitting within a percent level

Abstract

Kinematically forbidden channels can set the freeze-out dark matter (DM) relic abundance. These channels are described by DM annihilations into heavier states, which vanish at zero temperature limit, but occur at finite temperatures in the early Universe. For the case that the final state of the forbidden channel is scalar mediators that couple to Standard Model (SM) matter through mixing with the SM Higgs, the signals from DM-nucleon interactions and from mediator-related missing energy or displaced vertices could be detected by direct detections and particle physics experiments, respectively. We thus present a study on the simplest secluded vector dark matter model that can exhibit this scenario in the mass range from sub-GeV to TeV. The dark matter resides in the hidden sector, which is in thermal equilibrium with the SM before freeze-out. During freeze-out, the depletion of its density results from its annihilation into two heavier but metastable scalars, where the coupling can be determined by having the correct relic density and constrained by the perturbative unitarity bound. However, much of the allowed parameter space is insensitive to the mixing angle between the hidden scalar and SM Higgs. We find that a more significant mass splitting between DM and the mediator can be allowed only in the sub-GeV region. Moreover, the mass splitting in the TeV region is required to be within the percent level. This model of the forbidden DM interacting with SM particles through the scalar portal is testable in experiments.