New CAST limit on the axion–photon interaction

TLDR

Axions are hypothetical low‑mass particles that could explain dark matter and are expected to be produced abundantly in stellar interiors. CAST aims to test axion production by pointing a 9 T LHC test magnet at the Sun. Solar axions are converted to X‑ray photons in the magnetic field and detected by X‑ray detectors. CAST achieved a three‑fold improvement in signal‑to‑noise and set a 95 % confidence limit of 0.66×10⁻¹⁰ GeV⁻¹ on the axion–photon coupling, matching the tightest astrophysical bounds.

Abstract

Hypothetical low-mass particles, such as axions, provide a compelling explanation for the dark matter in the universe. Suchparticles are expected to emerge abundantly from the hot interior of stars. To test this prediction, the CERN Axion SolarTelescope (CAST) uses a 9 T refurbished Large Hadron Collider test magnet directed towards the Sun. In the strong magneticfield, solar axions can be converted to X-ray photons which can be recorded by X-ray detectors. In the 2013–2015 run, thanksto low-background detectors and a new X-ray telescope, the signal-to-noise ratio was increased by about a factor of three.Here, we report the best limit on the axion–photon coupling strength (0.66x10-10 GeV-1 at 95% confidence level) set byCAST, which now reaches similar levels to the most restrictive astrophysical bounds.