Centrality dependence of the expansion dynamics in Pb-Pb collisions at 158 A GeV c-1

Two-particle correlation functions of negatively charged hadrons from Pb-Pb collisions at 158 GeV c(-1)/nucleon have been measured by the WA97 experiment at the CERN SPS. A Coulomb correction procedure that assumes an expanding source has been implemented. Within the framework of an expanding thermalized source model the size and dynamical state of the collision fireball at freeze-out have been reconstructed as a function of the centrality of the collision. Less central collisions exhibit a different dynamics than central ones: both transverse and longitudinal expansion velocities are slower, the expansion duration is shorter and the system freezes out, showing smaller dimensions and higher temperature.