Large contribution of virtual Delbrück scattering to the emission of photons by relativistic nuclei in nucleus - nucleus and electron - nucleus collisions

Delbrück scattering is an elastic scattering of a photon in the Coulomb field of a nucleus via a virtual electron loop. The contribution of this virtual subprocess to the emission of a photon in the collision of ultra-relativistic nuclei Z1 Z2 → Z1 Z2 γ is considered. We identify the incoming virtual photon as being generated by one of the relativistic nuclei involved in the binary collision and the scattered photon as being emitted in the process. The energy and angular distributions of the photons are calculated. The discussed process has no infrared divergence. The total cross section obtained is 14 barn for Au–Au collisions at the RHIC collider and 50 barn for Pb–Pb collisions at the LHC collider. These cross sections are considerably larger than those for ordinary tree-level nuclear bremsstrahlung in the considered photon energy range me ≪ Eγ ≪ me γ, where γ is the Lorentz factor of the nucleus. Finally, photon emission in electron-nucleus collisions e Z → e Z γ is discussed in the context of the eRHIC option.