New solar axion search in CAST with ${}^4$He filling

The CERN Axion Solar Telescope (CAST) searches for $a\to \gamma$ conversion in the 9 T magnetic field of a refurbished LHC test magnet that can be directed toward the Sun. Two parallel magnet bores can be filled with helium of adjustable pressure to match the X-ray refractive mass $m_{\gamma }$ to the axion search mass $m_a$. After the vacuum phase (2003–2004), which is optimal for $m_a\lesssim 0.02$ eV, we used ${}^4$He in 2005–2007 to cover the mass range of 0.02–0.39 eV and ${}^3$He in 2009–2011 to scan from 0.39–1.17 eV. After improving the detectors and shielding, we returned to ${}^4$He in 2012 to investigate a narrow $m_a$ range around 0.2 eV (“candidate setting” of our earlier search) and 0.39–0.42 eV, the upper axion mass range reachable with ${}^4$He, to “cross the axion line” for the KSVZ model. We have improved the limit on the axion-photon coupling to $g_{a\gamma}< 1.47\times10^{-10} {\rm GeV}^{-1}$ (95% C.L.), depending on the pressure settings. Since 2013, we have returned to vacuum and aim for a significant increase in sensitivity.