A new upper limit on the axion-photon coupling with an extended CAST run with a Xe-based Micromegas detector

TL;DR

This paper reports an extended search for solar axions using a xenon-based detector at CERN, setting the most restrictive experimental limit on axion-photon coupling to date for low axion masses.

Contribution

It introduces a new upper limit on the axion-photon coupling by extending the CAST experiment with a xenon-based detector, providing technical insights for future IAXO detectors.

Findings

No axion signal detected during the extended run.

Improved upper limit on axion-photon coupling to 5.8×10^{-11} GeV^{-1}.

Most restrictive experimental limit for axion mass below 0.02 eV.

Abstract

Hypothetical axions provide a compelling explanation for dark matter and could be emitted from the hot solar interior. The CERN Axion Solar Telescope (CAST) has been searching for solar axions via their back conversion to X-ray photons in a 9-T 10-m long magnet directed towards the Sun. We report on an extended run with the IAXO (International Axion Observatory) pathfinder detector, doubling the previous exposure time. The detector was operated with a xenon-based gas mixture for part of the new run, providing technical insights for future detector configurations in IAXO. No counts are detected in the 95% signal-encircling region during the new run, while 0.75 are expected. The new data improve the axion-photon coupling limit to 5.8$\times 10^{-11}\,$GeV$^{-1}$ at 95% C.L. (for $m_a \lesssim 0.02$ eV), the most restrictive experimental limit to date.