Dark photon dark matter from an oscillating dilaton

TL;DR

This paper proposes a new mechanism for generating ultralight dark photon dark matter in the early Universe through a dilaton-like scalar field, enabling copious production even with small oscillations, and allowing for very light vector dark matter.

Contribution

It introduces a novel dilatonic coupling mechanism that efficiently produces ultralight dark photons, especially when their mass is half that of the dilaton, differing from axion-like models.

Findings

Dark photons can be produced copiously with small dilaton oscillations.

Ultralight vector dark matter with mass as low as 10^{-20} eV is possible.

The mechanism operates effectively in scenarios compatible with the cosmic microwave background.

Abstract

We present a mechanism for generating ultralight dark photon dark matter in the early Universe via a dilatonlike scalar field coupled to the dark photon's kinetic term. Energy is initially stored in the condensate of the dilaton, which resonantly produces dark photons when it begins oscillating in the early Universe. While similar scenarios with axion--dark-photon couplings require large coupling coefficients to fully populate the dark photon, the dilatonic coupling features a unique regime: When the dark photon's mass is half that of the dilaton, dark photons are copiously produced even when the dilaton undergoes small-amplitude oscillations. Scenarios consistent with the cosmic microwave background allow for ultralight vector dark matter with mass as light as $10^{-20}$ eV.