Long-hypothesised particles called axions could solve two problems in one strike: they could explain the puzzling symmetry properties of the strong force and they could make up the mysterious dark matter that permeates the cosmos. One of the newest detectors of the CAST experiment at CERN, RADES, has now joined the worldwide hunt for axions, searching for axions from the Milky Way's "halo" of dark matter and setting a limit on the strength of their interaction with photons. The results are described in a paper submitted for publication in the Journal of High Energy Physics.
One way of searching for axions from the Milky Way's dark-matter halo is to look for their conversion into photons in a "resonating cavity". If such axions surround and enter a resonating cavity that is placed in a strong magnetic field and resonates at a frequency corresponding to their mass, the chances of detecting them through their conversion into photons are increased.
Many experiments have used this search method and set limits on the interaction strength of axions with two photons in the case of small axion masses, mainly below 25 µeV (for comparison, the proton mass is 1 GeV). Searching for larger axion masses using this approach requires a smaller cavity resonating at a higher frequency, but the smaller volume of a smaller cavity decreases the chances of spotting the particles.
A workaround involves dividing the cavity into smaller cavities that resonate at a higher frequency and collectively don't result in a loss of cavity volume. This is exactly the concept behind the RADES detector, which was installed inside one of CAST's dipole magnet bores in 2018 and can search for axions from the Milky Way's dark-matter halo that have a mass of around 34.67 µeV.
Researchers are developing complementary approaches to searching for axions, and some have searched for larger-mass axions using new cavity designs and placed limits on their interaction strength with two photons. But the best limit so far for an axion mass of 34.67 µeV was placed by CAST's previous searches for axions from the Sun.
In its latest paper, the CAST team describes the results of the first RADES search for axions. Sifting through data taken for more than 100 hours within a period of 20 days in 2018, the team saw no signs of axions. However, the data places a limit on the interaction strength of axions with two photons in the case of axions with a mass of or close to 34.67 µeV - a limit that is more than 100 times more stringent than CAST's previous best limit for this mass.
"This result is a significant first step in the direct search for axions using dipole magnets," says RADES scientist Sergio Arguedas Cuendis. "And as far as axion searches go, it's one of the most stringent limits ever set for axions with masses above 25 µeV."
