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This article is about the hypothetical particle. Axion is also a genus of beetle of the ladybird family: see Axion (beetle)
The axion is a theoretical exotic particles postulated by Peccei-Quinn theory to resolve the strong-CP problem in quantum chromodynamics (QCD). The naïve first principles formulation of QCD without axions predicts that some strong interactions will violate CP-symmetry. This is never observed in practice, and the axion was postulated to be a particle (specifically a pseudo-Goldstone boson) associated with a new broken symmetry of nature, whose conservation is constructed to exclude all CP-violating terms from QCD.
Axions are predicted to have no charge, very low mass (10-6–10-2 eV/c2) and very low interaction cross-sections for strong and weak forces. Hence they are nearly invisible to ordinary matter, and cannot be excluded on the basis of current measurements, though they have never been observed.
Axion theories further suggest that axions were created abundantly during the big bang. Because of a unique coupling to the instanton field of the primordial universe (i.e., "misalignment mechanism"), an effective dynamical friction is created during the acquisition of mass following cosmic inflation. This robs all such primordial axions of their kinetic energy. Hence axion theories predict that the universe is filled with a very cold Bose-Einstein condensate of primordial axions. Depending on their mass, axions could plausibly explain the dark matter problem of cosmology. Observational studies to detect dark matter axions are underway, but they are not yet sufficiently sensitive to probe the mass regimes where axions would be expected to be found if they are the solution to the dark matter problem. Such studies have excluded the possibility of high mass axions.
It should be noted that the existence of axions is also a necessary component of string theory.
CERN's Axion Solar Telescope
CERN opened the CAST experiment in 2002, built to search for axions thought to be produced when thermal photons scatter from electrons and protons in the Sun's core. A detection of axions would represent evidence for a new branch of physics and partially could solve the dark matter problem. CAST uses a 10 meter long superconducting LHC magnet that produces a field of up to 9 Tesla (http://en.wikipedia.org/wiki/Tesla_%28unit%29) to convert axions to X-rays. X-ray detectors of different kind (pn-CCD, TPC, Micromegas) watch for converted axions at each end of the magnet.
The telescope observes the sun for three hours a day, one and a half hours at sunrise and one
and a half hours at sunset. The remaining time of the day the magnet is pointing away from the sun
to measure background. In 2003 and 2004 CAST has finished it's first data taking periods and the CAST collaboration is now preparing the magnet system for another data acquisition run. With the new setup CAST will be able to probe axion mass regions that are favored by theoretical models for the first time.
Links
November 24th news article by PhysicsWeb.org (http://physicsweb.org/articles/news/8/11/13/1)
CAST Experiment (http://www.cern.ch/cast)
CAST at MPI/MPE (http://cast.mppmu.mpg.de)
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