A TPC (time projection chamber) for axion searches in the CAST experiment at CERN

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 TPC in one of its three X-ray detectors, which looks for the "sunset" axions. The analysis of the 2003 data taken with this detector has shown no signal above the background, thus implying an upper limit to the axion-photon coupling of gagg < 1.55*10^-10 GeV^-1 at 95 % CL for ma < 0.02 eV. The stable operation of the experiment and the TPC during 2004 data taking allowed us to lower this value down to gagg < 1.29*10^-10 GeV^-1 at 95 % CL for ma < 0.02 eV. This work describes the analysis procedure for the data collected these two years.