Search for dark-photon dark matter in the SuperMAG geomagnetic field
  dataset
  In our recent companion paper [arXiv:2106.00022], we pointed out a novel
signature of ultralight kinetically mixed dark-photon dark matter. This
signature is a quasi-monochromatic, time-oscillating terrestrial magnetic field
that takes a particular pattern over the surface of the Earth. In this work, we
present a search for this signal in existing, unshielded magnetometer data
recorded by geographically dispersed, geomagnetic stations. The dataset comes
from the SuperMAG Collaboration and consists of measurements taken with
one-minute cadence since 1970, with $\mathcal{O}(500)$ stations contributing in
all. We aggregate the magnetic field measurements from all stations by
projecting them onto a small set of global vector spherical harmonics (VSH)
that capture the expected vectorial pattern of the signal at each station.
Within each dark-photon coherence time, we use a data-driven technique to
estimate the broadband background noise in the data, and search for excess
narrowband power in this set of VSH components; we stack the searches in
distinct coherence times incoherently. Following a Bayesian analysis approach
that allows us to account for the stochastic nature of the dark-photon
dark-matter field, we set exclusion bounds on the kinetic mixing parameter in
the dark-photon dark-matter mass range $2\times10^{-18}\,\text{eV} \lesssim
m_{A'} \lesssim 7\times10^{-17}\,\text{eV}$ (corresponding to frequencies
$6\times 10^{-4}\,\text{Hz}\lesssim f_{A'} \lesssim 2\times
10^{-2}\,\text{Hz}$). These limits are complementary to various existing
astrophysical constraints. Although our main analysis also identifies a number
of candidate signals in the SuperMAG dataset, these appear to either fail or be
in tension with various additional robustness checks we apply to those
candidates. We report no robust and significant evidence for a dark-photon
dark-matter signal in the SuperMAG dataset.
