Symmetry factor

A couple of years ago, an Italian experiment PVLAS produced results that gave some excitement to people searching for ultra-light particles, such as axion. What were they looking for?

Well, the experiment is looking at the effect of changing polarization of light propagating through a region of vacuum with magnetic field. There are a couple of interesting effects that can happen there. First, the vacuum state can become perturbed by the magnetic field. This is intersting, as it makes the vacuum itself behave like a birefringent medium. Of course, the phenomenon of birefringence has been known for centuries, for example in calcite crystals. It basically means that the medium has different index of refraction for different polarizations of light propagating in that medium (an interesting thing about this phenomenon is that it made lots of troubles for Newton’s corpuscular theory of light, giving way to wave theory, etc.). But it is just plain cool to see the same phenomenon for the vacuum, which happens due to quantum effects. And it can be done if one can send lineraly polarized light (from a laser) into a region of magnetic field and observe change of linear polarization of light to elliptical once light exits that region. To do so, the PVLAS experiment has a sensitive ellipsometer attempting to detect the small changes in the polarization state of light propagating through a 1 m long magnetic field region in vacuum. It is based on a high nesse Fabry-P erot cavity and a superconducting 5.5 T rotating magnet.

Second possible effect that can produce that change in polarization is even more beautiful. It could be due to the existence of a light neutral pseudoscalar or scalar particle coupled to two photons via the Primakoff effect. Now there is a nice candidate for that, an axion, a particle that is implied by some models that solve strong CP problem (some other things, like millicharged particles are also possible). So when PVLAS observed an effect of polarization change that was some orders of magnitude bigger than what they expected, it produced a small waterfall of papers from theorists and lots of discussions…

Well, unfortunately, those results turned out to be wrong. A paper from PVLAS (arXiv:0706.3419v1 [hep-ex]), which appeared on ArXiv today, announced that it was a detector effect. Here is their abstract:

” In 2006 the PVLAS collaboration reported the observation of an optical rotation generated in vacuum by a magnetic field. To further check against possible instrumental artifacts several upgrades to the PVLAS apparatus have been made during the last year. Two data taking runs, at the wavelength of 1064 nm, have been performed in the new configuration with magnetic field strengths of 2.3 T and 5.5 T. The 2.3 T field was chosen to avoid stray fields. The new observations do not show the presence of a rotation signal down to the levels of $1.2\cdot 10^{-8}$ rad @ 5.5 T and $1.0\cdot 10^{-8}$ rad @ 2.3 T (at 95% c.l.) with 45000 passes. In the same conditions no ellipticity signal was detected down to $1.4\cdot 10^{-8}$ @ 2.3 T (at 95% c.l.) whereas at 5.5 T a signal is still present. The physical nature of this ellipticity as due to an effect depending on $B^2$ can be excluded by the measurement at 2.3 T. These new results completely exclude our previously published magnetically induced vacuum dichroism results indicating that they were not of physical origin.These new results therefore also exclude the particle interpretation of the previous PVLAS results. Furthermore the background ellipticity at 2.3 T can be used to determine a limit on the total photon-photon scattering cross section of $\sigma_{\gamma\gamma} < 6 \cdot10^{-34}$ barn at 95% c.l.”

(the emphasis is mine). Too bad… but there is science for you…