Augury of Darkness: The Low-Mass Dark Z' Portal

TL;DR

This paper explores low-mass Z' portal models for Dirac and Majorana dark matter, analyzing collider, direct, and indirect detection constraints, and their potential to explain the Galactic center gamma-ray excess.

Contribution

It provides a comprehensive analysis of light Z' mediators in dark matter models, including collider bounds, scattering limits, and implications for gamma-ray excess.

Findings

Light Z' models are still constrained by current data.

Near-resonance scenarios can explain the Galactic center gamma-ray excess.

Constraints from collider and detection experiments significantly restrict low-mass Z' parameter space.

Abstract

Dirac fermion dark matter models with heavy $Z^{\prime}$ mediators are subject to stringent constraints from spin-independent direct searches and from LHC bounds, cornering them to live near the $Z^{\prime}$ resonance. Such constraints can be relaxed, however, by turning off the vector coupling to Standard Model fermions, thus weakening direct detection bounds, or by resorting to light $Z^{\prime}$ masses, below the Z pole, to escape heavy resonance searches at the LHC. In this work we investigate both cases, as well as the applicability of our findings to Majorana dark matter. We derive collider bounds for light $Z^{\prime}$ gauge bosons using the $CL_S$ method, spin-dependent scattering limits, as well as the spin-independent scattering rate arising from the evolution of couplings between the energy scale of the mediator mass and the nuclear energy scale, and indirect detection limits. We show that such scenarios are still rather constrained by data, and that near resonance they could accommodate the gamma-ray GeV excess in the Galactic center.