Chemical relaxation time of pions in hot hadronic matter
  We calculate characteristic time scales for chemical equilibration of pions in hot hadronic matter using an effective chiral Lagrangian. We find that inelastic processes involving the vector and axial vector mesons reduce the chemical equilibration time by a factor of $\sim 10$ compared to the result previously calculated in chiral perturbation theory. For a temperature of $T\sim 150 MeV$ we obtain a chemical relaxation time of $\tau_{ch} \simeq 10 fm/c$, 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 important for AGS-energies. We predict, that chemical freeze-out should take place at considerable higher temperatures $\Delta T \simeq 20 MeV$ than thermal freeze-out and that the hadronic phase would not sustain a pion chemical potential larger than $100 MeV$. 
Chungsik Song and Volker Koch