A search for the hypothetical axion

Has produced a new limit on the axion–photon interaction strength. The putative axion, a leading candidate for cosmological dark matter, could be produced in a two-photon interaction with an electric or magnetic field. Now, the CERN Axial Solar Telescope (CAST) collaboration has investigated how axions produced at the Sun interact with a laboratory magnetic field to back-convert into x rays. In the CAST experiment, which ran for about six months in 2003, a 10-m-long, 9-T magnet refurbished from the Large Hadron Collider followed the Sun like a telescope. It was outfitted with x-ray detectors and an x-ray telescope recovered from the German space program. No axions were seen, but for lightweight axions of 0.02 eV or less, the data analysis improved the previous state-of-the-art laboratory limit on the axion-photon interaction strength by a factor of five. The CAST group expects further improvement after analyzing their 2004 data. (K. Zioutas et al., Phys. Rev. Lett. 94, 121301, 2005 http://dx.doi.org/10.1103/PhysRevLett.94.121301 .)