Axions ruled out by PVLAS
Jul 2, 2007
The existence of a hypothetical particle called the axion has been put into further doubt now that the team that first claimed its discovery has failed to reproduce their results. Physicists working on the PVLAS experiment in Italy say that the tiny rotation in the polarization of laser light that they reported last year does not support the existence of axions, but rather is an artefact related to how the experiment had been performed (arXiv:0706.3419v1).
Axions are ultralight particles that were first postulated in the 1970s to resolve a discrepancy between experimental findings and the theory of the strong force. In March 2006, the PVLAS team shone a laser beam through a strong magnetic field of 5.5 T in a vacuum and saw that the beam's polarization rotated slightly. At the time many physicists thought that this was due to an ultralight particle coupling with photons in the beam, and so heralded it as the first glimpse of the axion.
Now, the PVLAS team has repeated the original experiment at two different magnetic field strengths. While the rotation was again observed at the original field of 5.5 T, no effect was seen at 2.3 T – leading the team to conclude that the rotation is an instrumental effect related to the magnetic field strength.
The latest news from Italy should come as a relief to physicists who believe that axions could make up dark matter. This is because the PVLAS axion appeared to couple too strongly to light to be a suitable candidate for dark matter.
The null result also puts PVLAS line with an experiment at CERN called CAST, which has been trying to convert solar photons into axions in a 10-m long test magnet. CAST has found no evidence for axions at the coupling strength implied by the 2006 PVLAS result.
Physicists are now gearing up to search for axions by studying gamma rays from a distant quasar that will soon be passing through the intense magnetic field of the sun. Such observations can be made every October when the Sun comes between the Earth and the quasar 3C 279. During this alignment, a small number of the quasar's gamma rays could be converted into axions at the far surface of the sun. The axions should then travel through the Sun unhindered, only to be converted back into gamma rays when they emerge on the near side. Unconverted gamma rays, however, would be blocked by the sun and therefore any gamma rays from the quasar that are detected on Earth would be seen as evidence of axions.
About the author
Hamish Johnston is editor of Physics Web