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The CERN Axion Solar Telescope (CAST) is searching for axions produced in the Sun's core by the Primakoff process. CAST is using a decommissioned Large Hadron Collider (LHC) test magnet where axions could be converted back into X-rays with energies up to 10 keV. Analysis of the 2003 data showed no signal above background implying an upper limit for the axion-photon coupling constant gagg < 1.16 X 10 ^-10 GeV exp -1 at 95% C.L. for ma . 0.02 eV [1]. The higher quality 2004 data is presently under analysis. CAST Phase II is scheduled to start in late 2005. This will be the first step in extending CAST's sensitivity to axion rest masses up to ~ 1 eV.
In non-hadronic axion models, which have a tree-level axion-electron interaction, the Sun produces a strong axion flux by bremsstrahlung, Compton scattering, and axiorecombination, the "BCA processes." Based on a new calculation of this flux, including for the first time axio-recombination, we derive limits on the axion-electron Yukawa coupling gae and axion-photon interaction strength ga using the CAST phase-I data (vacuum phase). For ma <~ 10 meV/c2 we find ga gae < 8.1 × 10−23 GeV−1 at 95% CL. We stress that a next-generation axion helioscope such as the proposed IAXO could push this sensitivity into a range beyond stellar energy-loss limits and test the hypothesis that white-dwarf cooling is dominated by axion emission.
