Using the Earth's magnetic field to hunt for axions and dark photons

Dark matter remains elusive, despite astronomers believing it constitutes around a quarter of the universe's total energy. Its true nature has eluded scientists for decades, with two main candidates being ultralight axions and dark photons. Currently, conventional axion searches involve converting them into photons using strong magnets, but this confines the experiment to a controlled space.

The research team from Kyoto University, Hiroshima University, and Nihon University discovered they could leverage Earth's magnetic field, which spans a scale unattainable by laboratories. By harnessing the Earth–ionosphere cavity, they amplified electromagnetic waves within the mass range they were investigating. Theoretical framework limitations meant they couldn't predict frequencies above 1 Hz, but they overcame this by developing a new model accounting for atmospheric conductivity.

This model predicted amplification near 8 Hz and extended predictions up to 30 Hz. When analyzing a decade's worth of geomagnetic field data from 2012 to 2022, the team expected axion-origin signals to vary by location, with Southeast Asia showing the strongest signals. On the other hand, dark photon signals would appear nearly identical across locations.

Utilizing their framework, the team examined geomagnetic data for a steady, narrow-frequency signal expected from dark matter over long timescales, followed by statistical analysis. They then applied the same approach to dark photons, which can generate electromagnetic waves without a magnetic field, and searched the same data set for their unique signature.

The team's method tightened constraints on axion coupling to light by about 100 times compared to previous ground-based experiments, rivaling astrophysical X-ray observation constraints like those from Chandra and NuSTAR. While no definitive dark matter signal emerged, the dark photon analysis uncovered several potential candidates that could indicate dark matter presence, though these need further confirmation.

This study's theoretical framework is expected to pave the way for a new era in dark matter searches, as Atsushi Nishizawa et al. concluded in their paper published in Progress of Theoretical and Experimental Physics.