On the route for detecting vacuum birefringence at an XFEL combined with a PW-class laser

Vacuum birefringence as a clear variant of light-by-light scattering is a feasible experiment at an XFEL combined with a PW-class laser. Such experiment would measure the birefringence of vacuum – induced by the giant electromagnetic field in the focus of the PW laser – directly by ultra-sensitive X-ray polarimetry. In this way, the effect depends on real photons of externally controllable beams, complementary to other effects where photon-photon couplings are in play. The HIBEF user consortium at the European XFEL is aiming for such experiment. We will give a status update on the progress made in recent beamtimes, concerning a) the polarimeter extinction for a well-collimated (laser-like) X-ray beam, b) the X-ray optics to be used inside the polarimeter, c) self-seeded operation of the XFEL and d) assuring best focusing/intensity verification.