Freeze-out time in ultrarelativistic heavy ion collisions from Coulomb effects in transverse pion spectra

The influence of the nuclear Coulomb field on transverse spectra of ${\pi }^{+}$ and ${\pi }^{-}$ measured in Pb+Pb reactions at $158A$ GeV has been investigated. Pion trajectories are calculated in the field of an expanding fireball. The observed enhancement of the ${\pi }^{-}/{\pi }^{+}$ ratio at small momenta depends on the temperature and transverse expansion velocity of the source, the rapidity distribution of the net positive charge, and mainly the time of the freeze-out.