Search for solar axions: the CAST experiment

Hypothetical axion‐like particles with a two‐photon interaction would be produced in the sun by the Primakoff process. In a laboratory magnetic field they would be transformed into X‐rays with energies of a few keV. The CAST experiment at CERN is using a decommissioned LHC magnet as an axion helioscope in order to search for these axion‐like particles. The analysis of the 2003 data showed no signal above the background, thus implying an upper limit to the axion‐photon coupling of gaγ < 1.16 × 10−10 GeV−1 at 95% CL for ma ≲ 0.02 eV. The stable operation of the experiment during 2004 data taking allowed us to lower down this parameter to a preliminary value of gaγ < 0.9 × 10−10 GeV−1.