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 mesons reduce the chemical equilibration time by a factor of ∼10 compared to the result previously calculated in chiral perturbation theory. For a temperature of T∼160 MeV we obtain a chemical relaxation time of τch≅7 fm/c, which is comparable 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 at considerably higher temperatures ΔT≅40 MeV than thermal freeze-out and that the hadronic phase would not sustain a pion chemical potential larger than 50 MeV. copyright 1997 The American Physical Society