Figure 1: Diagrams illustrating the QED LbyL interaction processes and the equivalent photon approximation.
From: Evidence for light-by-light scattering in heavy-ion collisions with the ATLAS detector at the LHC
a, Diagrams for Delbrück scattering (left), photon splitting (middle) and elastic LbyL scattering (right). Each cross denotes external field legs, for example, an atomic Coulomb field or a strong background magnetic field. b, Illustration of an ultra-peripheral collision of two lead ions. Electromagnetic interaction between the ions can be described as an exchange of photons that can couple to form a given final state X. The flux of photons is determined from the Fourier transform of the electromagnetic field of the ion, taking into account the nuclear electromagnetic form factors.
