In a paper published in Nature Physics, the CERN Axion Solar Telescope (CAST) has reported important new exclusion limits on coupling of axions to photons. Axions are hypothetical particles that interact very weakly with ordinary matter and therefore are candidates to explain dark matter. They were postulated decades ago to solve the “strong CP” problem in the Standard Model (SM), which concerns an unexpected time-reversal symmetry of the nuclear forces. Axion-like particles, unrelated to the strong-CP problem but still viable dark-matter candidates, are also predicted by several theories of physics beyond the SM, notably string theory.
A variety of Earth- and space-based observatories are searching possible locations where axions could be produced, ranging from the inner Earth to the galactic centre and right back to the Big Bang. CAST looks for solar axions using a “helioscope” constructed from a test magnet originally built for the Large Hadron Collider. The 10 m-long superconducting magnet acts like a viewing tube and is pointed directly at the Sun: solar axions entering the tube would be converted by its strong magnetic field into X-ray photons, which would be detected at either end of the magnet. Starting in 2003, the CAST helioscope, mounted on a movable platform and aligned with the Sun with a precision of about 1/100th of a degree, has tracked the movement of the Sun for an hour and a half at dawn and an hour and a half at dusk, over several months each year.
In the latest work, based on data recorded between 2012 and 2015, CAST finds no evidence for solar axions. This has allowed the collaboration to set the best limits to date on the strength of the coupling between axions and photons for all possible axion masses to which CAST is sensitive. The limits concern a part of the axion parameter space that is still favoured by current theoretical predictions and is very difficult to explore experimentally, allowing CAST to encroach on more restrictive constraints set by astrophysical observations. “Even though we have not been able to observe the ubiquitous axion yet, CAST has surpassed even the sensitivity originally expected, thanks to CERN’s support and unrelenting work by CASTers,” says CAST spokesperson Konstantin Zioutas. “CAST’s results are still a point of reference in our field.”
The experience gained by CAST over the past 15 years will help physicists to define the detection technologies suitable for a proposed, much larger, next-generation axion helioscope called IAXO. Since 2015, CAST has also broadened its research at the low-energy frontier to include searches for dark-matter axions and candidates for dark energy, such as solar chameleons.