Searching for Solar Axions with CAST

The CAST (CERN Axion Solar Telescope) experiment is the worldwide most sensitive helioscope searching for solar axions by their conversion into photons inside the magnet pipes of a decommissioned LHC dipole magnet. The expected converted photons can be measured by four independent X-ray detectors installed at each end of the two magnet bores. These detectors are an X-ray telescope and a Micromegas detector on the western end of the CAST magnet and on the eastern end, a Time Projection Chamber (TPC) was installed until 2006, covering the two ends of the magnet pipes. In 2007 the TPC has been replaced by two upgraded Micromegas detectors, based on newest technology. The first phase of the CAST experiment (2003-2004) with vacuum in the conversion region resulted in an upper limit on the axion-photon coupling that supersedes the best limits derived from stellar energy-loss arguments. Since 2005, CAST entered in its second phase where it operates with a low Z buffer gas in the conversion region in order to extend the sensitivity of the experiment to higher axion rest masses. During 2005 and 2006, 4He was used to search for an axion signal with axion rest masses in the range from 0.02 eV up to 0.4 eV. In order to search for even higher axion rest masses up to 1.2 eV the experiment was adapted for 3He data taking with an upgraded gas system in 2007. The data taking with 3He inside the magnet pipes has started successfully in March 2008.