A 3.55 keV Line from Exciting Dark Matter without a Hidden Sector

TL;DR

This paper proposes a new model of Exciting Dark Matter (XDM) that explains the 3.55 keV line and Galactic Center gamma-ray excess without requiring a hidden sector, using a pseudoscalar mediator with large Yukawa couplings.

Contribution

It introduces a non-hidden sector XDM model with a pseudoscalar mediator, capable of explaining multiple astrophysical signals while satisfying experimental constraints.

Findings

Model can generate the 3.55 keV line and Galactic Center gamma-ray excess.

Parameter space avoids constraints from colliders and direct detection.

No need for a hidden sector in the dark matter model.

Abstract

Models in which dark matter particles can scatter into a slightly heavier state which promptly decays to the lighter state and a photon (known as eXciting Dark Matter, or XDM) have been shown to be capable of generating the 3.55 keV line observed from galaxy clusters, while suppressing the flux of such a line from smaller halos, including dwarf galaxies. In most of the XDM models discussed in the literature, this up-scattering is mediated by a new light particle, and dark matter annihilations proceed into pairs of this same light state. In these models, the dark matter and mediator effectively reside within a hidden sector, without sizable couplings to the Standard Model. In this paper, we explore a model of XDM that does not include a hidden sector. Instead, the dark matter both up-scatters and annihilates through the near resonant exchange of a $\mathcal{O}(10^2)$ GeV pseudoscalar with large Yukawa couplings to the dark matter and smaller, but non-neglibile, couplings to Standard Model fermions. The dark matter and the mediator are each mixtures of Standard Model singlets and $SU(2)_W$ doublets. We identify parameter space in which this model can simultaneously generate the 3.55 keV line and the gamma-ray excess observed from the Galactic Center, without conflicting with constraints from colliders, direct detection experiments, or observations of dwarf galaxies.