Relic Challenges for Vector-Like Fermions as Connectors to a Dark Sector

TL;DR

This paper examines the challenges of connecting dark sectors with the Standard Model via vector-like fermions, showing minimal models are mostly ruled out by direct detection, and proposing solutions involving Majorana masses and lepton mixing.

Contribution

It introduces a simple model of vector-like fermion connectors to dark sectors and explores mechanisms to evade experimental constraints.

Findings

Minimal models are generally excluded by direct dark matter searches.

Introducing a Majorana mass can reduce scattering on nuclei.

Mixing with SM leptons allows decay of connectors, satisfying cosmological constraints.

Abstract

New dark sectors consisting of exotic fields that couple only very feebly to the Standard Model (SM) have strong theoretical motivation and may be relevant to explaining the abundance of dark matter (DM). An important question for such sectors is how they connect to the SM. For a dark sector with a new gauge interaction, a natural connection arises from heavy vector-like fermions charged under both the visible and dark gauge groups. The gauge charges of such fermions imply that one or more of them is stable in the absence of additional sources of dark symmetry breaking. A generic challenge for such connectors is that they can produce too much dark matter or interact too strongly with nuclei if they were ever thermalized in the early universe. In this paper we study this challenge in a simple connector theory consisting of new vector-like electroweak doublet and singlet fermions that also transform under the fundamental representation of a new (Abelian) gauge force, and we show that these connectors in their minimal form are almost always ruled out by existing direct DM searches. To address this challenge, we investigate two solutions. First, we study mitigating scattering on nuclei by introducing a Majorana mass term for the singlet. And second, we investigate a mixing with SM leptons that allows the connectors to decay while remaining consistent with cosmological tests and searches for charged lepton flavor violation. Both solutions rely on the presence of a dark Higgs field with a specific charge.