π−π+ ratio in heavy ion collisions: Coulomb effect or chemical equilibration?

We calculate the π−π+ ratio for Pb + Pb at CERN/SPS energies and for Au + Au at BNL/AGS energies using a (3+1) dimensional hydrodynamical model. Without consideration of Coulomb effect an enhancement of this ratio at low mt is found compatible with that observed in these experiments. Our calculations are based on previous (3+1) dimensional hydrodynamical simulations (HYLANDER), which described many other aspects of experimental data. In this model the observed enhancement is a consequence of baryon and strangeness conservation and of chemical equilibration of the system and is caused by the decay of produced hyperons, which leads to a difference in the total number of positive and negative pions as well. Based on the same approach, we also present results for the π−π+ ratio for S + S (CERN/SPS) collisions, where we find a similar effect. The absence of the enhancement of the π−π+ ratio in the S + S data presented by the NA44 Collaboration, if confirmed, could indicate that chemical equilibration has not yet been established in this reaction.