Hidden axion dark matter decaying through mixing with QCD axion and the 3.5 keV X-ray line

TL;DR

This paper explores how hidden axions, mixing with QCD axions, could decay into photons to explain the 3.5 keV X-ray line, with implications for dark matter properties and non-Gaussian primordial perturbations.

Contribution

It proposes a novel scenario where hidden axions decay via mixing with QCD axions, accounting for the X-ray line and affecting dark matter and primordial perturbation characteristics.

Findings

Hidden axion decay can explain the 3.5 keV X-ray line.

Long-lived hidden axion dark matter is feasible at intermediate decay constants.

Primordial density perturbations of hidden axions may be highly non-Gaussian.

Abstract

Hidden axions may be coupled to the standard model particles through a kinetic or mass mixing with QCD axion. We study a scenario in which a hidden axion constitutes a part of or the whole of dark matter and decays into photons through the mixing, explaining the 3.5 keV X-ray line signal. Interestingly, the required long lifetime of the hidden axion dark matter can be realized for the QCD axion decay constant at an intermediate scale, if the mixing is sufficiently small. In such a two component dark matter scenario, the primordial density perturbations of the hidden axion can be highly non-Gaussian, leading to a possible dispersion in the X-ray line strength from various galaxy clusters and near-by galaxies. We also discuss how the parallel and orthogonal alignment of two axions affects their couplings to gauge fields.