Researcher
研究者名
Taruya, Atsushi
Overview
Kyoto, Japan -- Dark matter's existence is all but certain -- astronomers believe it makes up about a quarter of the universe's total energy content -- yet its true identity has eluded us for decades. Two of the leading candidates for dark matter are the hypothetical particles ultralight axions and dark photons, which in the range studied here would be some 19 to 21 orders of magnitude lighter than the electron.
Conventional axion searches tend to involve converting them into photons with the help of strong laboratory magnets. However, research in a laboratory inherently limits the space over which such a field can be applied. A collaborative team of researchers from Kyoto University, Hiroshima University, and Nihon University realized that, by contrast, Earth's own magnetic field spans a scale no laboratory could match.
"We asked ourselves whether we could use the Earth itself as a giant detector in the search," says corresponding author Atsushi Taruya. "The Earth-ionosphere cavity acts as a natural resonator that amplifies electromagnetic waves right around the mass range we wanted to probe."
However, existing theory could only handle frequencies below 1 Hz, leaving the rest of this range unpredicted. This inspired the team to construct a new theoretical framework that accounts for the atmosphere's electrical conductivity, both showing that the Earth-ionosphere cavity amplifies signals near 8 Hz and thereby extending reliable predictions up to about 30 Hz. They expected the axion-origin signals to vary by location, with the highest in Southeast Asia, whereas dark photon signals should look nearly the same everywhere.
Building on their new framework, the team analyzed about a decade's worth of geomagnetic field data from 2012 to 2022 from the British Geological Survey's Eskdalemuir Observatory. They removed artificial noise, then searched for the steady, narrow-frequency signal that dark matter is expected to produce over long timescales, and followed this with statistical analysis. The team then extended the same theoretical framework to dark photons, which unlike axions generate electromagnetic waves even without a magnetic field present, and searched the same dataset for their distinct signature.
By treating the entire Earth as a giant detector for a specific range of axion masses, the limits the team set on how strongly axions couple to light were roughly 100 times tighter than the previous best from a ground-based experiment. Those results even rival constraints from astrophysical X-ray observations like Chandra and NuSTAR, which themselves rely on certain theoretical assumptions. Mysteriously, the dark photon analysis also turned up several signal candidates that could potentially originate from dark matter, although their true nature has yet to be confirmed.
For now, the identity of dark matter is still a mystery. But the theoretical framework developed in this study is expected to underpin a new phase of dark matter searches.
