Scalar field dark matter with spontaneous symmetry breaking and the $3.5$ keV line

TL;DR

This paper proposes a scalar field dark matter model with spontaneous symmetry breaking that explains the 3.5 keV X-ray line and accounts for dark matter abundance through interactions with the Higgs field, avoiding isocurvature issues during inflation.

Contribution

It introduces a novel scalar field dark matter scenario with Higgs interactions and non-minimal gravity coupling, explaining the 3.5 keV line and dark matter stability.

Findings

Scalar field can account for dark matter abundance.

Model predicts decay into photon pairs matching the 3.5 keV line.

Compatible with cosmological and astrophysical constraints.

Abstract

We show that the present dark matter abundance can be accounted for by an oscillating scalar field that acquires both mass and a non-zero expectation value from interactions with the Higgs field. The dark matter scalar field can be sufficiently heavy during inflation, due to a non-minimal coupling to gravity, so as to avoid the generation of large isocurvature modes in the CMB anisotropies spectrum. The field begins oscillating after reheating, behaving as radiation until the electroweak phase transition and afterwards as non-relativistic matter. The scalar field becomes unstable, although sufficiently long-lived to account for dark matter, due to mass mixing with the Higgs boson, decaying mainly into photon pairs for masses below the MeV scale. In particular, for a mass of $\sim 7$ keV, which is effectively the only free parameter, the model predicts a dark matter lifetime compatible with the recent galactic and extragalactic observations of a 3.5 keV X-ray line.