Photon regeneration from pseudoscalars at X-ray laser facilities

Recently, the PVLAS Collaboration reported an anomalously large rotation of the polarization of light in the presence of a magnetic field in vacuum. As a possible explanation, they consider the existence of a light spin-zero particle coupled to two photons. We propose here a method of independently testing this result using a high-energy photon regeneration experiment (the x-ray analogue of "invisible light shining through walls") using the synchrotron x rays from a free-electron laser. With such an experiment the region of parameter space implied by PVLAS could be probed in a matter of minutes.