Electromagnetic electron-positron pair production in heavy-ion collisions with impact parameter zero

We present a calculation of the second-order process of electron-positron pair creation by the electromagnetic fields of two relativistic heavy ions for impact parameter b zero. Total probabilities as well as differential ones are presented. A comparison with the results of the double equivalent-photon approximation (DEPA) shows explicitly the inapplicability of this approximation in the case of total probabilities. We discuss the creation of pairs with large invariant mass. The DEPA results are too high, which can be traced back to the mass singularity of the photon cross section at very small angles, giving a discrepancy, whereas the agreement between our result and DEPA is good for larger angles