Scale-invariant scalar field dark matter through the Higgs portal

TL;DR

This paper explores a scale-invariant scalar field coupled to the Higgs as a dark matter candidate, analyzing its dynamics, stability, decay, and potential astrophysical signatures, including explaining the 3.5 keV X-ray line.

Contribution

It introduces a scale-invariant Higgs-portal scalar field model for dark matter, detailing its behavior before and after electroweak symmetry breaking and potential observable signals.

Findings

Stable for positive Higgs coupling, unstable for negative coupling

Decay into photons possible with very long lifetime

A 7 keV scalar can explain the 3.5 keV X-ray line

Abstract

We discuss the dynamics and phenomenology of an oscillating scalar field coupled to the Higgs boson that accounts for the dark matter in the Universe. The model assumes an underlying scale invariance such that the scalar field only acquires mass after the electroweak phase transition, behaving as dark radiation before the latter takes place. While for a positive coupling to the Higgs field the dark scalar is stable, for a negative coupling it acquires a vacuum expectation value after the electroweak phase transition and may decay into photon pairs, albeit with a mean lifetime much larger than the age of the Universe. We explore possible astrophysical and laboratory signatures of such a dark matter candidate in both cases, including annihilation and decay into photons, Higgs decay, photon-dark scalar oscillations and induced oscillations of fundamental constants. We find that dark matter within this scenario will be generically difficult to detect in the near future, except for the promising case of a 7 keV dark scalar decaying into photons, which naturally explains the observed galactic and extra-galactic 3.5 keV X-ray line.