Scalar Dark Matter Candidates -- Revisited

TL;DR

This paper reevaluates light scalar dark matter candidates in the MeV to GeV range, incorporating recent experimental constraints and astrophysical bounds, and finds most scenarios are excluded except near resonant annihilation conditions.

Contribution

It provides updated limits from collider, astrophysical, and cosmological data on light scalar dark matter models, especially considering new experimental advances and the resonant annihilation regime.

Findings

Most scalar dark matter models are excluded by combined constraints.

Resonant annihilation near on-shell mediators remains a viable scenario.

Galactic 511 keV excess cannot be explained by these particles under current constraints.

Abstract

We revisit the possibility of light scalar dark matter, in the MeV to GeV mass bracket and coupled to electrons through fermion or vector mediators, in light of significant experimental and observational advances that probe new physics below the GeV-scale. We establish new limits from electron colliders and fixed-target beams, and derive the strength of loop-induced processes that are probed by precision physics, among other laboratory probes. In addition, we compute the cooling bound from SN1987A, consider self-scattering, structure formation, and cosmological constraints as well as the limits from dark matter-electron scattering in direct detection experiments. We then show that the combination of constraints largely excludes the possibility that the galactic annihilation of these particles may explain the long-standing INTEGRAL excess of 511 keV photons as observed in the galactic bulge. As caveat to these conclusions we identify the resonant annihilation regime where the vector mediator goes nearly on-shell.