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.

11 pages including 2 figures

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