New solar axion search using the CERN Axion Solar Telescope with 4He 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 4He in 2005-2007 to cover the mass range of 0.02-0.39 eVand ³He in 2009-2011 to scan from 0.39 to 1.17 eV. After improving the detectors and shielding, we returned to 4He 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 4He, to "cross the axion line" for the KSVZ model. We have improved the limit on the axion-photon coupling to gaγ < 1.47 x 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. [ABSTRACT FROM AUTHOR]