Kinetic Mixing, Dark Photons and Extra Dimensions II: Fermionic Dark Matter

TL;DR

This paper explores 5-D models of fermionic dark matter interacting via a dark photon, introducing mechanisms for light Dirac dark matter and relic density production without co-annihilation, extending previous scalar dark matter models.

Contribution

It extends 5-D kinetic mixing models to fermionic dark matter, proposing new mechanisms for light Dirac dark matter and relic density generation without co-annihilation.

Findings

A new 5-D mechanism allows light Dirac dark matter (~100 MeV) to evade CMB and 21 cm constraints.

Majorana dark matter can achieve relic density through direct s-channel annihilation without co-annihilation.

Kaluza-Klein excitations of the dark photon play a crucial role in annihilation processes.

Abstract

Extra dimensions can be very useful tools when constructing new physics models. Previously, we began investigating toy models for the 5-D analog of the kinetic mixing/vector portal scenario where the interactions of bulk dark matter with the brane-localized fields of the Standard Model are mediated by a massive $U(1)_D$ dark photon also living in the bulk. In that setup, where the dark matter was taken to be a complex scalar, a number of nice features were obtained such as $U(1)_D$ breaking by boundary conditions without the introduction of a dark Higgs field, the absence of potentially troublesome SM Higgs-dark singlet mixing, also by boundary conditions, the natural similarity of the dark matter and dark photon masses and the decoupling of the heavy gauge Kaluza-Klein states from the Standard Model. In the present paper we extend this approach by examining the more complex cases of Dirac and Majorana fermionic dark matter. In particular, we discuss a new mechanism that can occur in 5-D (but not in 4-D) that allows for light Dirac dark matter in the $\sim 100$ MeV mass range, even though it has an $s$-wave annihilation into Standard Model fields, by avoiding the strong constraints that arise from both the CMB and 21 cm data. This mechanism makes use of the presence of the Kaluza-Klein excitations of the dark photon to extremize the increase in the annihilation cross section usually obtained via resonant enhancement. In the Majorana dark matter case, we explore the possibility of a direct $s$-channel dark matter pair-annihilation process producing the observed relic density, due to the general presence of parity-violating dark matter interactions, without employing the usual co-annihilation mechanism which is naturally suppressed in this 5-D setup.