Swampland Bounds on Dark Sectors

TL;DR

This paper applies Swampland principles to constrain dark matter models, ruling out certain ultra-light particles and setting bounds on dark photon parameters, thereby linking quantum gravity constraints with experimental searches.

Contribution

It introduces new Swampland bounds on dark sector particles and dark photons, connecting quantum gravity constraints with observable parameter spaces.

Findings

Ultra-light particles are in the Swampland, regardless of abundance.

Constraints on dark photon masses and kinetic mixings affect upcoming detection experiments.

Certain dark matter models, like freeze-in and 21-cm radio models, are disfavored by Swampland bounds.

Abstract

We use Swampland principles to theoretically disfavor regions of the parameter space of dark matter and other darkly charged particles that may exist. The Festina Lente bound, the analogue of the Weak-Gravity conjecture in de Sitter, places constraints on the mass and charge of dark particles, which here we show cover regions in parameter space that are currently allowed by observations. As a consequence, a broad set of new ultra-light particles are in the Swampland, independently of their cosmic abundance, showing the complementarity of Quantum Gravity limits with laboratory and astrophysical studies. In parallel, a Swampland bound on the UV cutoff associated to the axion giving a St\"{u}ckelberg photon its longitudinal mode translates to a new constraint on the kinetic mixings and masses of dark photons. This covers part of the parameter space targeted by upcoming dark-photon direct-detection experiments. Moreover, it puts astrophysically interesting models in the Swampland, including freeze-in dark matter through an ultra-light dark photon, as well as radio models invoked to explain the 21-cm EDGES anomaly.