Relic Abundance of Dark Photon Dark Matter

TL;DR

This paper introduces a novel mechanism where an axion transfers energy to dark photons via tachyonic instability, producing the correct relic abundance of dark photon dark matter across a wide mass range, including extremely light masses.

Contribution

It proposes a new production mechanism for dark photon dark matter involving axion oscillations and tachyonic instability, supported by numerical lattice simulations.

Findings

Dark photons can account for dark matter over a broad mass range.

The mechanism is efficient when dark photon and axion masses are within a few orders of magnitude.

Numerical simulations confirm the viability of the proposed production process.

Abstract

We present a new mechanism for producing the correct relic abundance of dark photon dark matter over a wide range of its mass, extending down to $10^{-20}\,\mathrm{eV}$. The dark matter abundance is initially stored in an axion which is misaligned from its minimum. When the axion starts oscillating, it efficiently transfers its energy into dark photons via a tachyonic instability. If the dark photon mass is within a few orders of magnitude of the axion mass, $m_{\gamma'}/m_a = {\cal O}(10^{-3} - 1)$, then dark photons make up the dominant form of dark matter today. We present a numerical lattice simulation for a benchmark model that explicitly realizes our mechanism. This mechanism firms up the motivation for a number of experiments searching for dark photon dark matter.