Magnetometer networks as dark-matter detectors

Distributed magnetometer networks hold exceptional promise as tools to search for fundamental physics beyond the Standard Model. Two well-motivated types of ultralight dark-matter (DM) candidates, dark photons and axion-like particles, are known to generate electromagnetic effects in certain circumstances. In this talk, I will discuss recent work in which my collaborators and I pointed out the existence of a novel signal of this type for each of these DM candidates. The signal is a small (but larger than expected) oscillating magnetic-field pattern that appears across the entire surface of the Earth, driven by the DM field. The signal is highly phase-coherent and has a frequency set by the DM mass. The specific signal pattern generated depends on the DM candidate. I will also discuss recent work of ours that implemented searches for the signal patterns that arise for each of these DM candidates in an existing dataset maintained by the SuperMAG Collaboration. These data consist of unshielded magnetometer readings taken at O(500) geographically distributed geomagnetic ground stations. The data have one-minute time resolution, with the earliest data taken in 1970. Our search finds no robust evidence for either DM candidate. However, we place the first direct exclusion bounds on dark-photon DM in the mass-range from 2e-18 eV to 7e-17 eV; these limits are complementary to existing astrophysical bounds. For axion-like DM, we place limits in the same mass range that at some masses rival existing laboratory constraints on axions. I will mention future plans to extend these searches.