Search for Sub-eV Mass Solar Axions by the CERN Axion Solar Telescope with (3)He Buffer Gas

The CERN Axion Solar Telescope (CAST) has extended its search for solar axions by using 3He as a buffer gas. At T 1⁄4 1:8 K this allows for larger pressure settings and hence sensitivity to higher axion masses than our previous measurements with 4He. With about 1 h of data taking at each of 252 different pressure settings we have scanned the axion mass range 0:39 eV & ma & 0:64 eV. From the absence of excess x rays when the magnet was pointing to the Sun we set a typical upper limit on the axion-photon coupling of ga & 2:3 1010 GeV1 at 95% C.L., the exact value depending on the pressure setting. Kim-Shifman-Vainshtein-Zakharov axions are excluded at the upper end of our mass range, the first time ever for any solar axion search. In the future we will extend our search to ma & 1:15 eV, comfortably overlapping with cosmological hot dark matter bounds.