Size of fireballs created in high energy heavy ion collisions as inferred from Coulomb distortions of pion spectra

We compute the Coulomb effects produced by an expanding highly charged fire ball on the momentum distribution of pions. We compare our results to data on Au + Au at 11.6A GeV from E866 at the BNL AGS and to data on Pb + Pb at 158A GeV from NA44 at the CERN SPS. We conclude that the distortion of the spectra at low transverse momentum and midrapidity can be explained in both experiments by the effect of the large amount of participating charge in t he central rapidity region. By adjusting the fireball expansion velocity to match the average transverse momentum of protons, we find a best fit when the fireball radius is about 10 fm, as determined by the moment when the pi ons undergo their last scattering. This value is common to both the AGS and CERN experiments. [S0556-2813(99)04406-4].