Dark matter remains one of the greatest mysteries in modern physics, accounting for most of the universe’s mass while remaining invisible to current detection methods.
Researchers at the University of Oxford, the UK Science and Technology Facilities Council (STFC), and other institutions are now exploring a potential key to this puzzle: axions, hypothetical particles that could be a major component of dark matter. To detect them, they are using the European X-ray Free Electron Laser (XFEL), the world’s most powerful X-ray laser, located in Hamburg, Germany.
Axions are tiny, low-mass particles that have been theorized to exist but have never been directly observed. The XFEL facility features a 3.4-kilometer (2.11-mile) superconducting linear accelerator and photon beamlines capable of producing 27,000 ultrashort X-ray flashes per second. This high-frequency output is critical for the experiment.
The experiment is based on a principle known as “light shining through walls.” X-rays are directed through germanium crystals with a strong internal electric field that mimics the effects of a powerful magnetic field, potentially converting photons into axions. A titanium sheet blocks the photons, but if axions exist, they should pass through and then convert back into photons on the other side, allowing detection.
According to the research team, this method provides sensitivity to axions comparable to other accelerator-based experiments. Lead author Dr. Jack Halliday, an experimental plasma physicist at STFC, stated that this study highlights XFEL’s versatility in tackling fundamental physics questions.
The experiment lays the foundation for more refined searches targeting axions predicted by Quantum Chromodynamics (QCD), the theory describing the strong force that binds quarks and gluons and is essential for matter’s stability.
In particle physics, one long-standing mystery is why neutrons, despite containing charged quarks, do not exhibit an electric dipole moment. This suggests an unknown balancing mechanism, which axions could explain. If confirmed, axions would not only solve this puzzle but also challenge the Standard Model, the current framework of fundamental particles and forces.
Axions are also strong dark matter candidates due to their weak interactions and low mass, fitting the expected properties of dark matter particles. Unlike ordinary matter, dark matter does not interact with light, making it undetectable by traditional telescopes.
Previous studies have suggested that axions could form clouds around neutron stars and potentially emit detectable radio signals. If confirmed, these signals could offer crucial insights into dark matter’s true nature.
The findings of this latest experiment were published in the journal Physical Review Letters.