CHEMICAL RELAXATION-TIME OF PIONS IN HOT HADRONIC MATTER

We calculate characteristic time scales for the chemical equilibration of pions in hot hadronic matter using an effective chiral Lagrangian. We find that inelastic processes involving vector and axial vector me sons reduce the chemical equilibration time by a factor of similar to 10 compared to tile result previously calculated in chiral perturbatio n theory. For a temperature of T similar to 160 MeV we obtain a chemic al relaxation time of tau(ch)similar or equal to 7 fm/c, which is comp arable with typical time scales for a hadronic system generated in SPS -energy heavy-ion collisions. The effect of baryons is also estimated and found to be negligible for SPS energies but might be important for AGS energies. We predict that chemical freeze-out should take place a t considerably higher temperatures Delta T similar or equal to 40 MeV than thermal freeze-out and that the hadronic phase would not sustain a pion chemical potential larger than 50 MeV.