An improved limit on the axion–photon coupling from the CAST experiment

We have searched for solar axions or similar particles that couple to two photons by using the CERN Axion Solar Telescope (CAST) setup with improved conditions in all detectors. From the absence of excess X-rays when the magnet was pointing to the Sun, we set an upper limit on the axion-photon coupling of 8.8 x 10^{-11} GeV^{-1} at 95% CL for m_a <~ 0.02 eV. This result is the best experimental limit over a broad range of axion masses and for m_a <~ 0.02 eV also supersedes the previous limit derived from energy-loss arguments on globular-cluster stars.