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Hypothetical axion-like particles with a two-photon interaction would be produced in the Sun by the Primakoff process. In a laboratory magnetic field (“axion helioscope”) they would be transformed into X-rays with energies of a few keV. Using a decommissioned LHC test magnet, CAST has been running for about 6 months during 2003. The first results from the analysis of these data are presented here. No signal above background was observed, implying an upper limit to the axion-photon coupling g_aγ < 1.16 × 10⁻¹⁰GeV⁻¹ at 95% CL for m_a ≲ 0.02 eV. This limit is comparable to the limit from stellar energy-loss arguments and considerably more restrictive than any previous experiment in this axion mass range.

The CAST experiment is being mounted at CERN. It will make use of a decommissioned LHC test magnet to look for solar axions through its conversion into photons inside the magnetic field. The magnet has a field of 9 Tesla and length of 10 m and is installed in a platform which allows to move it ±8° vertically and ±40° horizontally. According to these numbers we expect a sensitivity in axion-photon coupling g_aγγ ≲ 5 × 10⁻¹¹GeV⁻¹ for m_a ≲ 10⁻²eV, and with a gas filled tube g_aγγ ≲ 10⁻¹⁰GeV⁻¹ for m_a ≲ 2 eV.

A fast photon-detection system for the detector RICH-1 of the COMPASS Experiment at CERN SPS is in operation since the 2006 run. It is based on the use of Multi-Anode Photomultipliers (MAPMTs) coupled to individual fused silica lens telescopes and fast read-out electronics. It has been designed taking into account the high photon flux in the central region of the detector and the high rate requirements of the COMPASS Experiment. We present the photon-detection design and construction, together with its characterization and measured performances based on the data collected in 2006.

Solar axions can be produced in the Sun via the so-called Primakoff effect. The CERN Axion Solar Telescope (CAST) uses an LHC prototype magnet of about 9 T to reconvert these axions into photons.

The magnet is able to follow the Sun for about 3 hours per day. Three different X-Ray detectors are mounted on its ends to detect photons from axion-to-photon conversion: a Time Projection Chamber (TPC), a MICROMEGAS (MICROMEsh GAseous Structure) and a Charge Coupled Device (CCD). For the CCD an X-ray focusing device is used to improve the signal-to-background ratio significantly.

With the completion of CAST'S first phase, the current limits on the coupling constant g_aγ for axion masses up to 0.02 eV have been improved. In its second phase, CAST extends the axion mass range by filling the magnet with a buffer gas. Masses up to about 0.4 eV have already been covered and thus the experiment is entering the regions favored by axion models. This paper will present the status of CAST'S second phase.