If the axion exists, it escapes the Sun faster than light reaches it—taking virtually no time to cross the solar system because it interacts with almost nothing. In Hamburg, a new experiment now aims to directly detect this hypothetical particle for the first time: BabyIAXO at the German Electron Synchrotron (DESY), equipped with a superconducting ten-meter magnet that tracks the Sun daily and may solve one of physics' most persistent mysteries.
The puzzle behind the experiment
About 27 percent of the universe consists of dark matter. Observations of galaxy rotation, gravitational lensing, and the cosmic microwave background confirm this. Yet this matter makes itself known only through gravity, emits no light, and matches no known particle. For decades, WIMPs (weakly interacting massive particles) were the leading candidate. Multiple experiments have searched their predicted parameter space almost completely—and found nothing.
Axions are now the most promising remaining candidate. Roberto Peccei and Helen Quinn proposed the particle in 1977 not to solve the dark matter problem, but another: the strong CP problem. The strong nuclear force, described by quantum chromodynamics, should theoretically violate CP symmetry—distinguishing matter from its mirror image. Yet it demonstrably does not. The Peccei-Quinn mechanism adds a new symmetry to theory and explains this absence. The byproduct is an extremely light, electrically neutral particle: the axion. That it also works as a cold dark matter candidate was a later, fortunate discovery of cosmology.
The helioscope principle
At the Sun's core, temperatures reach about 15 million degrees Celsius. According to theory, axions form there through the Primakoff process: thermal photons interact with the strong electric field of solar plasma nuclei and partially convert into axions. Because axions interact so weakly, they escape the Sun unimpeded and stream outward in all directions.
An axion helioscope reverses this process. The experimental magnet provides a strong magnetic field. Axions flowing through its bore can convert back into X-ray photons via the inverse Primakoff effect. These photons are focused by a downstream X-ray telescope onto a tiny detector area. If no signal appears, no axions were found but coupling limits tighten. If it does appear, a discovery.
The CAST experiment (CERN Axion Solar Telescope) tested this principle from 2003 to 2021 at CERN. It used a recycled LHC dipole magnet, ten meters long, nine tesla field strength, tracking the Sun daily for roughly 90 minutes at sunrise and sunset. CAST found no axions but set new limits: axion-photon coupling must lie below a precisely defined threshold.
BabyIAXO: Hamburg takes over
BabyIAXO is CAST's successor and prototype for the International Axion Observatory (IAXO). Its location is HERA South Hall at DESY Hamburg, where the HERA electron-proton storage ring once operated.
The heart is a superconducting toroidal magnet built from aluminum-stabilized Nb-Ti/Cu Rutherford cable, ten meters long, generating a magnetic field of roughly two tesla. Two bore channels, each 70 centimeters in diameter, run through the system. Each bore houses a complete detection line with X-ray optics and detector. BabyIAXO thus measures two independent signal channels simultaneously—something CAST could not do. Like its predecessor, the entire apparatus rotates daily with the Sun.
The magnet's commissioning is scheduled for 2026. First scientific data is expected no earlier than 2029, once all detector lines are installed and calibrated. The international collaboration includes groups from CERN, Spain, France, and Germany. From Germany, the Max Planck Institute for Physics in Munich and the Physics Institute of the University of Bonn participate. DESY provides infrastructure and experimental hall.
2029: First data run or strongest argument for IAXO
BabyIAXO is explicitly designed as a dual experiment: on one hand, a fully functional helioscope with its own discovery potential; on the other, a test bed for the later IAXO. That instrument would include a 20-meter magnet with eight detection lines and be roughly 10,000 times more sensitive than CAST. IAXO's location remains open.
If BabyIAXO finds axions, two of the most stubborn open questions in fundamental physics are answered at once: the nature of dark matter and the strong CP problem. That would be the most significant discovery in particle physics since the Higgs boson in 2012. If BabyIAXO finds nothing, it is no failure but the strongest argument for IAXO. Limits tighten, the remaining search space shrinks, and somewhere within it, theory says, the axion must hide.
Dark matter is not speculation. Its gravitational effect has been measured across hundreds of galaxies. What is missing is the particle. In HERA South Hall in Hamburg, it may leave its first signal.