Impact-parameter dependence of multiple lepton-pair production from electromagnetic fields

In relativistic heavy-ion collisions, the strong Lorentz-contracted electromagnetic fields are capable of producing copious numbers of lepton pairs through the two-photon mechanism. Monte Carlo techniques have been developed that allow the exact calculation of production by this mechanism when a semiclassical approximation is made for the motion of the two ions. Here, we develop a hybrid Monte Carlo technique that enables us to calculate the impact-parameter dependence of the two-photon mechanism for lepton-pair production, and by using this result we obtain the probability distribution for multiple-pair production as a function of impact parameter. Computations are performed for S+Au and Pb+Pb systems at 200 and 160 A GeV, respectively. We also compare our results with the equivalent-photon approximation and elucidate the differences.