Freeze-Out Time in Ultrarelativistic Heavy Ion Collisions from Coulomb Effects in Transverse Pion Spectra

Abstract

The influence of the nuclear Coulomb field on transverse spectra of \(\pi^+\) and \(\pi^-\) measured in \(Pb+Pb\) reactions at 158 A 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.