Proving the existence of axion
The axion is a hypothetical elementary particle postulated by the Peccei-Quinn theory in 1977 to resolve the strong-CP problem in quantum chromodynamics (QCD). The suggested properties of axion are that it is very light and interacts very weakly with matter. This interaction of axion with matter is so weak that so far no-one has managed to detect it.
Physicists believe that high-energy photons and other neutral cosmic rays (excluding neutrinos) coming from far off galaxies to earth should not be able to travel inter-galactic distances because they would be absorbed by the universe’s opaque background of microwaves. Yet it seems these photons and cosmic rays do travel these inter-galactic distances because they are detected on earth.
Malcolm Fairbairn of King’s College London and his group are proposing that cosmic-ray photons could reach Earth from distant galaxies by temporarily converting into axions, which can bypass the microwave background without absorption. They are proposing this based on the fact that they have found a correlation between where detected cosmic rays originate and where photons and axions are more likely to “mix”.
In the experiments being performed by Fairbairn’s group, a laser is shone onto a wall in the presence of a magnetic field. If some of the laser’s photons covert into axions, they could travel freely through the wall, revert to photons and then be detected on the other side. They have coined these experiments as the “shining light through the wall” experiments.
According to Fairbairn and colleagues they hope that, on a cosmic scale, the “wall” could be provided by the microwave background, and the magnetic field could be provided by galaxies.
Just as in the “shining light through the wall” experiments, in order to see if the “shining light through the universe” effect exists, Fairbairn’s group has performed a statistical analysis of neutral cosmic rays of energies above 1018 eV recorded by the High Resolution Fly’s Eye (HiRes) detector in Utah. Previous studies had already shown a correlation between the arrival locations of these cosmic rays and the known locations of highly luminous “active” nuclei of distant galaxies. But Fairbairn’s group has shown that there is an additional correlation with the likely profile of our own galaxy’s magnetic field, which determines the probability of photon-to-axion conversions. The researchers say the likelihood of the correlation occurring by coincidence is just 2.4% (arXiv:0901.4085 ).
But Fairbairn himself admits that they are still far from the truth because there is no way of knowing how many of the neutral cosmic rays detected by High Resolution Fly’s Eye (HiRes) are actually photons, as all it can detect are the subsequent showers when the rays collide with the Earth’s atmosphere. Also there are doubts about the effectiveness of the photon-to-axion conversion mechanism at such high energies.
Similar to the correlation obtained by Fairbairn’s group the work done by Hooper, Pasquale Serpico of CERN and Melanie Simet of the University of Chicago, found a correlation for lower energy photons last year (Phys. Rev. D 77 063001).
February 9, 2009