CAST reaches milestone but keeps on searching

After eight years of searching for the emission of a dark matter candidate particle, the axion, from the Sun, the CERN Axion Solar Telescope (CAST) has fulfilled its original physics programme.


Members of the CAST collaboration in July, together with dipole-based helioscope.

CAST, the world’s most sensitive axion helioscope, points a recycled prototype LHC dipole magnet at the Sun at dawn and dusk, looking for the conversion of axions to X-rays. It incorporates four state-of-the-art X-ray detectors: three Micromegas detectors and a pn-CCD imaging camera attached to a focusing X-ray telescope that was recovered from the German space programme (see CERN Courier April 2010). 

Over the years, CAST has operated with the magnet bores - the location of the axion conversion - in different conditions: first in vacuum, covering axion masses up to 20 meV/c2, and then with a buffer gas (4He and later 3He) at various densities, finally reaching the goal of 1.17 eV/c2 on 22 July. While a direct solar axion signal remains elusive, the experiment has set the most restrictive experimental limit on the axion-photon coupling strength for rest masses that include the theoretically and cosmologically motivated range from μeV/c2 to eV/c2. At the same time, the collaboration has gained very valuable experience in low-background detectors (< 10 keV) and ultra cold (around 1.8 K) gas systems.

After scheduled maintenance, CAST will resume data taking in 2012 with improved sensitivity for solar axions (hot dark matter). The experiment will also expand its physics horizon, searching for paraphotons (the “hidden sector”), chameleons (dark energy), while exploring the ambitious possibility of searching for relic axions (cold dark matter) in an, as yet, inaccessible rest mass range of around 0.1 -1 meV/c2.


by CERN Courier (september 2011 issue)