Scale-invariant scalar field dark matter through the Higgs portal

TL;DR

This paper proposes a scale-invariant scalar field coupled to the Higgs as a dark matter candidate, explaining the 3.5 keV X-ray line and exhibiting unique cosmological dynamics, but challenging to test experimentally.

Contribution

It introduces a novel scale-invariant Higgs-portal scalar field model for dark matter that explains the 3.5 keV line and its cosmological behavior.

Findings

Scalar field behaves as dark radiation before electroweak transition.

Scalar field acts as non-relativistic matter after electroweak transition.

Model can account for the 3.5 keV X-ray line with a 7 keV mass.

Abstract

We introduce an oscillating scalar field coupled to the Higgs that can account for all dark matter in the Universe. Due to an underlying scale invariance of this model, the dark scalar only acquires mass after the electroweak phase transition. We discuss the dynamics of this dark matter candidate, showing that it behaves like dark radiation until the Electroweak phase transition and like non-relativistic matter afterwards. In the case of a negative coupling to the Higgs field, the scalar gets a vacuum expectation value after the electroweak phase transition and may decay into photons, although being sufficiently long-lived to account for dark matter. We show that, within this scenario, for a mass of 7 keV, the model can explain the observed galactic and extra-galactic 3.5 keV X-ray line. Nevertheless, it will be very difficult to probe this model in the laboratory in the near future.