New Upper Limit on the Axion-Photon Coupling with an Extended CAST Run with a Xe-Based Micromegas Detector

Hypothetical axions provide a compelling explanation for dark matter and could be emitted from the hot solar interior. The CERN Axion Solar Telescope has been searching for solar axions via their back conversion to x-ray photons in a 9-T 10-m long magnet directed toward the Sun. We report on an extended run with the 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 configurations. No counts were detected in the 95% signal-encircling region during the new run, while 0.75 were expected. The new data improve the axion-photon coupling limit to $5.8×{10}^{-11}\text{}\text{}{\mathrm{GeV}}^{-1}$ at 95% CL (for ${m}_{a}\lesssim 0.02\text{}\text{}\mathrm{eV}$), the most restrictive experimental limit to date.