Relic abundance of dark photon dark matter

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 10−20eV. 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, mγ′/ma=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.