Exact $Z^2$ scaling of pair production in the high-energy limit of   heavy-ion collisions
  The two-center Dirac equation for an electron in the external electromagnetic field of two colliding heavy ions in the limit in which the ions are moving at the speed of light is exactly solved and nonperturbative amplitudes for free electron-positron pair production are obtained. We find the condition for the applicability of this solution for large but finite collision energy, and use it to explain recent experimental results. The observed scaling of positron yields as the square of the projectile and target charges is a result of an exact cancellation of a nonperturbative charge dependence and holds as well for large coupling. Other observables would be sensitive to nonperturbative phases. 
B. Segev and J. C. Wells (Institute for Theoretical Atomic and   Molecular Physics, Harvard-Smithsonian Center for Astrophys...