$Z$ boson mediated dark matter beyond the effective theory

TL;DR

This paper examines a singlet-doublet model of Majorana fermion dark matter mediated by the Z boson, analyzing its viability against direct detection constraints and its potential for future experimental probing.

Contribution

It revisits the singlet-doublet model focusing on the Higgs blind spot, showing its compatibility with direct detection bounds and identifying regions accessible to future experiments.

Findings

Future experiments can probe most of the parameter space.

The model reproduces dark matter relic density in certain regions.

A small coannihilating region remains challenging to detect.

Abstract

Direct detection bounds are beginning to constrain a very simple model of weakly interacting dark matter---a Majorana fermion with a coupling to the $Z$ boson. In a particularly straightforward gauge-invariant realization, this coupling is introduced via a higher-dimensional operator. While attractive in its simplicity, this model generically induces a large $\rho$ parameter. An ultraviolet completion that avoids an overly large contribution to $\rho$ is the singlet-doublet model. We revisit this model, focusing on the Higgs blind spot region of parameter space where spin-independent interactions are absent. This model successfully reproduces dark matter with direct detection mediated by the $Z$ boson, but whose cosmology may depend on additional couplings and states. Future direct detection experiments should effectively probe a significant portion of this parameter space, aside from a small coannihilating region. As such, $Z$-mediated thermal dark matter as realized in the singlet-doublet model represents an interesting target for future searches.