Nonabelian dark matter models for 3.5 keV X-rays

TL;DR

This paper investigates nonabelian dark matter models with GeV-scale masses and eV-scale splittings to explain the 3.55 keV X-ray line, exploring decay and scattering mechanisms, constraints, and potential tests.

Contribution

It introduces nonabelian dark matter models with natural eV-scale splittings and analyzes their decay and scattering processes related to the 3.55 keV X-ray line.

Findings

Models are constrained by direct detection and CMB data.

Upscattering can produce the X-ray line with velocity-dependent signatures.

Potential for future tests in fixed target experiments and astrophysical observations.

Abstract

A recent analysis of XXM-Newton data reveals the possible presence of an X-ray line at approximately 3.55 keV, which is not readily explained by known atomic transitions. Numerous models of eV-scale decaying dark matter have been proposed to explain this signal. Here we explore models of multicomponent nonabelian dark matter with typical mass ~ 1-10 GeV (higher values being allowed in some models) and eV-scale splittings that arise naturally from the breaking of the nonabelian gauge symmetry. Kinetic mixing between the photon and the hidden sector gauge bosons can occur through a dimension-5 or 6 operator. Radiative decays of the excited states proceed through transition magnetic moments that appear at one loop. The decaying excited states can either be primordial or else produced by upscattering of the lighter dark matter states. These models are significantly constrained by direct dark matter searches or cosmic microwave background distortions, and are potentially testable in fixed target experiments that search for hidden photons. We note that the upscattering mechanism could be distinguished from decays in future observations if sources with different dark matter velocity dispersions seem to require different values of the scattering cross section to match the observed line strengths.