Observability of Coulomb-assisted quantum vacuum birefringence

We consider the scattering of an x-ray free-electron laser (XFEL) beam on the superposition of a strong magnetic field Bext with the Coulomb field E ext of a nucleus with charge number Z . In contrast to Delbrück scattering (Coulomb field only), the magnetic field Bext introduces an asymmetry (i.e., polarization dependence) and renders the effective interaction volume quite large, while the nuclear Coulomb field facilitates a significant momentum transfer Δk. For a field strength of Bext = 106 T (corresponding to an intensity of order 1022 W / cm2) and an XFEL frequency of 24 keV, we find a differential cross section dσ/dΩ ∼10−25 Z2 / (Δk)2 in forward direction for one nucleus. Thus, this effect might be observable in the near future at facilities such as the Helmholtz International Beamline for Extreme Fields at the European XFEL.