Exploring Earth’s ionosphere for dark matter conversion signals

Dark matter, predicted to account for most of the universe’s mass, remains highly elusive. Physicists have been searching for various particles that could be promising dark matter candidates, such as dark photons and axions, focusing on signatures associated with their presence or interactions with other particles under specific conditions.

Researchers at the University of Geneva, CERN and Sapienza University of Rome recently explored the possibility of searching for dark matter particles by focusing on their predicted conversion into low-frequency radio waves in the Earth’s ionosphere. Their paper, published in Physical Review Letters, could open new possibilities for future dark matter searches, by highlighting a parameter space that has so far been unexplored.

“Many researchers have considered the resonant conversion of ultralight dark matter (ULDM) candidates, such as axions or dark photons, into standard model photons in astrophysical environments,” Carl Beadle, first author of the paper, told Phys.org.

“This was, for instance, explored in neutron stars, the solar corona and even on planets in our solar system like Jupiter. We asked whether such a signal could be provided to us from our local, naturally occurring plasma: the ionosphere. Given that it is extremely well monitored and understood, it seemed to us like a very good place to look.”

Within theoretically motivated models, a large fraction or the entirety of dark matter may be comprised of axions or dark photons. The idea proposed by Beadle and his colleagues is that these particles can convert into regular photons within the ionosphere, which would make them detectable using affordable antennae on Earth.

“The resonant conversion occurs if the mass of the dark matter particles coincides in value with a frequency characterizing the plasma,” explained Beadle. “One can think of this ‘plasma frequency’ as being like the number density of free electrons in the plasma, and because this density changes with height in the ionospheric plasma, this coincidence can occur if the mass of the dark matter happens to fall somewhere in the correct range.”

The researchers computed the conversion rate for the signal they predicted, accounting for various effects that could weaken it. They then compared photons exhibiting this signal to noise (i.e., unrelated photons) that could reach a potential antenna, to estimate the potential of their approach for detecting dark matter axions or dark photons in a real-world experiment.

Their findings suggest that an electrically small dipole antenna could detect their predicted signal. This hypothesis could be tested in future experiments.

“It should be stressed that we think this experiment would be reasonably cheap to produce and run, and it allows for a large part of theory space to be experimentally probed,” said Beadle. “There is also less astrophysical uncertainty associated with this proposal because the ionosphere is so well known and due to its location.”

The recent study by this team of researchers introduces a new avenue for exploring uncharted regions of the dark matter parameter space. Beadle and his colleagues have already started working with other experimental physicists, to plan future dark matter searches based on their predictions.

“We have been in contact with various experimental groups and there is pre-existing data to sort through to look for our signal,” added Beadle. “There are other researchers interested in constructing our proposal. We are now very excited to get involved with experimentalists to perform tests, while also working on improving the calculations for the signal.”

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