Perfecting the Solar axion hunt at CERN Print E-mail
With a new upgrade allowing searches for higher mass axions, the CERN axion Solar Telescope (CAST) is entering new axion territory this autumn.

CAST is a CERN Astroparticle physics experiment, what searches for axions, particles that according to theory are abundantly produced in the sun’s core. Axions were originally proposed to solve the strong CP problem, a serious problem in the prevailing theoretical framework of Particle Physics but were soon recognized to be one of the most important dark matter candidate particles. So far this new particle remains elusive, with all efforts to detect it producing null results.


 The CAST experiment. Credit: © CERN

The CAST experiment. Credit: © CERN

CAST, perfecting an innovative technique, uses a powerful superconducting magnet to convert the axions into X-rays photons via their interaction with the strong magnetic field. The magnet tracks the sun for about three hours a day, during sunrise and sunset. Special detectors are employed to look for excess axion-induced X-rays expected during sun tracking in the energy range 1-10 keV.

The detectors used so far are: a detector based on CCD's (which are similar to the ones used in digital cameras), a Micromegas detector (the Micromesh Gaseous detector is a new device containing gas and utilizing a fine metallic mesh to setup a strong field needed to produce a measurable electronic pulse for every passage of an X-ray photon or a charged particle) and a TPC (the Time Projection Chamber belongs to an older and widely used technology of gaseous detectors for photons and charged particles).

The experiment, in operation since 2003, has already published the most stringent limits for the axion-photon interaction strength
(Journal of Cosmology and Astroparticle Physics - 0704) in the case of low mass axions with a mass below 0.02 eV. During 2006, phase II of the experiment started with the addition of small amounts of 4He gas in the vacuum space of the magnet, making CAST sensitive to higher mass axions.


The detection of excessive X-rays at a certain gas pressure will signal the presence of specific mass axions and will allow the experimenters to measure the strength of their coupling to photons.
The 4He-gas-addition trick works up to axion masses of about 0.4 eV. Above that, one has to use 3He, a much more expensive gas. During 2007, the cryogenic system was modified for the use of the new gas with the aim of reaching axion masses up to 1.2 eV. The experimenters also decided to replace the sunset TPC X-ray detector and the sunrise Micromegas detector with three state-of-the-art bulk-technology Micromegas, one novel design X-ray focusing device (one exists already in the CCD sunrise axion line), and more cosmic ray shielding, in order to further improve the sensitivity of the experiment.
The hunt for solar axions with the CAST experimental apparatus is picking up with plans to implement further searches for much lower energy axions or axion-like particles, even in the visible photon energy range.
The new Micromegas line for sunrise axions. Credit: © CAST collaboration
The new Micromegas line for sunrise axions.
Credit: © CAST collaboration




Submitted by Georgios Fanourakis


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