New CAST Limit on the Axion-Photon Interaction

During 2003–2015, the CERN Axion Solar Telescope (CAST) has searched for $a\to \gamma$ conversion in the 9 T magnetic field of a refurbished LHC test magnet that can be directed toward the Sun. In its final phase of solar axion searches (2013–2015), CAST has returned to evacuated magnet pipes, which is optimal for small axion masses. The absence of a significant signal above background provides a world leading limit of $g_{a\gamma} < 0.66 \times 10^{-10} {\rm GeV}^{-1}$(95$m_a \lesssim 0.02$ eV. Compared with the first vacuum phase (2003–2004), the sensitivity was vastly increased with low-background x-ray detectors and a new x-ray telescope. These innovations also serve as pathfinders for a possible next-generation axion helioscope.