  Coulomb corrections to perturbation theory for producing electron-positron
pairs in ultrarelativistic heavy ion collisions are considered in a
part-analytical, part-numerical approach. Production probabilities are reduced
from perturbation theory with increasing charge of the colliding heavy ions, as
has been previously argued in the literature. It is shown here that the
reduction from perturbation theory comes from the appropriate physical spatial
cutoff of the electromagnetic potentials arising from the colliding
ultrarelativistic heavy ions.
