2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/191007The influence of the nuclear Coulomb field on transverse spectra of $π^+$ and $π^-$ 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 $π^-/π^+$ 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 figuresNuclear TheoryHigh Energy Physics - Phenomenologytext