Revisiting unitarity corrections for electromagnetic processes in   collisions of relativistic nuclei
  Unitarity corrections to several electromagnetic processes in collisions of relativistic heavy nuclei are considered. They are due to the unitarity requirement for the S-matrix and correspond to the exchange of light-by-light scattering block between colliding nuclei. We obtain improved results for the corrections to e+e- and mu+mu- pair production, as well as new results for unitarity corrections to the production of photons via virtual Compton and virtual Delbruck scattering. These corrections can be numerically large; e.g., the mu+mu- pair production cross section is reduced by about 50% and the nuclear bremsstrahlung by about 15 to 20%. 
U. D. Jentschura, K. Hencken and V. G. Serbo