Observability of Coulomb-assisted quantum vacuum birefringence
N. Ahmadiniaz, Michael Bußmann, T. E. Cowan, Alexander Debus, T. Kluge, Ralf Schützhold
Abstract
We consider the scattering of an x-ray free-electron laser (XFEL) beam on the superposition of a strong magnetic field ${\mathbf{B}}_{\mathrm{ext}}$ with the Coulomb field ${\mathbf{E}}_{\mathrm{ext}}$ of a nucleus with charge number $Z$. In contrast to Delbr\"uck scattering (Coulomb field only), the magnetic field ${\mathbf{B}}_{\mathrm{ext}}$ 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 $\mathrm{\ensuremath{\Delta}}\mathbf{k}$. For a field strength of ${B}_{\mathrm{ext}}={10}^{6}\text{ }\text{ }\mathrm{T}$ (corresponding to an intensity of order ${10}^{22}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$) and an XFEL frequency of 24 keV, we find a differential cross section $d\ensuremath{\sigma}/d\mathrm{\ensuremath{\Omega}}\ensuremath{\sim}{10}^{\ensuremath{-}25}\text{ }{Z}^{2}/(\mathrm{\ensuremath{\Delta}}\mathbf{k}{)}^{2}$ 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.