Shining Light through a Wall with Axions

Transparency of the Sun to Gamma Rays Due to Axionlike Particles

Malcolm Fairbairn, Timur Rashba, and Sergey Troitsky

PRL 98, 201801 (2007)

URL: http://link.aps.org/abstract/PRL/v98/e201801

Interesting stuff! The paper centers around the following logical deduction:

If axions allow light to pass through walls, then axions would allow gamma rays to pass through the sun.


• What is an axion?

An axion is a hypothetical pseudoscalar particle. (It behaves like a scalar function, except it changes sign under spatial reflections.) Axions cn couple to photons in a magnetic field. They are expected --- if they exist --- tobe light and weakly coupled.

Although the coupling between axions and photons is weak, it would have observable consequences. In a magnetic field, some light would be converted to axions, and interaction between left and right circularly polarized light would be mediated by axions. In QED, there is no coupling between the two opposite polarizations.


• Have axions been observed?

Maybe. An experiment called PVLAS detected a shift in the polarization of a laser passing through a strong magnetic field. The results are consistent with an axion mass of 1 meV and an inverse coupling of 100 TeV. However, an experiment called CAST claims to have ruled out a coupling that strong --- even though 100 TeV is a rather weak, unless you're a string theorist!


• How might one look for axions?

One method is that employed by the people at PVLAS: to search for polarization shifts in a strong magnetic field. However, the coupling of axions and photons offers a second, more exotic search technique: look for the transmission of light through an opaque object.

A strong magnetic field on one side of the object would convert some photons to axions. The axions would be able to pass through the object while the photons would not. A second strong magnetic field on the other side of the barrier would convert some of the transmitted axions back into photons, so it would appear that some light passed through the object. In short, if axions exist, then two strong magnetic fields would allow you to shine your flashlight through a rock!


• What's the sun got to do with any of this?

The group of physicists who wrote this paper believe that astronomical observations could determine the existence of axions. They have constructed a simple analytic model and numerically investigated a more detailed model, both of which predict the magnetic fields of the sun should allow the transmission of gamma rays through the sun via the axion-photon conversion process described above. The sun is normally opaque to gamma rays, and it does not produce a significant amount of gamma rays, so there should be little background interference.

The requirement for this type of observation is that a distant gamma ray source be identified whose line of sight from earth (or a space-based gamma ray detector) is eclipsed by the sun. If there are no axions, or if the coupling is too weak, the gamma ray source will disappear behind the sun. If gamma rays are still detected when the sun is between the source and the detector, it will provide strong evidence for the existence of axions.

There are gamma ray detectors on earth. Unfortunately for the authors, only one gamma ray source has been studied whose line of sight is obstructed by the sun. The statistics on this single source are not good enough to rule either way on the existence of axions, however the predicted and observed fluxes agree rather well. The flux definitely did not drop to zero when the sun obstructed the source.

In coming years, astronomical observations like those discussed by the authors may resolve the contradictory findings of PVLAS and CAST. The authors also suggest that decommissioned magnets from particle accelerators may allow for observation of axion-assisted photon tunneling in earth-based labs. No matter where the experimental data comes from, our knowledge of axions is going to increase quite a lot in the coming years.