Photons and Dark Photons Through Breit-Wheeler Processes
  A variant of quantum electrodynamics coupled to a dark photon through a kinetic mixing is studied. The analogous of the light-light diagram becomes the conversion process $\gamma'\gamma' \to \gamma \gamma$ and an expression for the differential cross section is estimated. For high energies beams, as in LHC, this differential cross section could be measurable and its magnitude would be typically similar to the total cross section of neutrinos, {\it i.e.} $\sim 10^{-50}\,{\text{m}}^2$. 
Ariel Arza, Jorge Gamboa and Natalia Tapia
hep-ph