New solar axion search using the CERN Axion Solar Telescope with He 4 filling

The CERN Axion Solar Telescope (CAST) searches for a→γ 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 mγ to the axion search mass ma. After the vacuum phase (2003–2004), which is optimal for ma≲0.02  eV, we used He4 in 2005–2007 to cover the mass range of 0.02–0.39 eV and He3 in 2009–2011 to scan from 0.39 to 1.17 eV. After improving the detectors and shielding, we returned to He4 in 2012 to investigate a narrow ma range around 0.2 eV (“candidate setting” of our earlier search) and 0.39–0.42 eV, the upper axion mass range reachable with He4, to “cross the axion line” for the KSVZ model. We have improved the limit on the axion-photon coupling to gaγ<1.47×10−10  GeV−1 (95% C.L.), depending on the pressure settings. Since 2013, we have returned to the vacuum and aim for a significant increase in sensitivity.