  One surprising result in relativistic heavy-ion collisions is that the
abundance of various particles measured in experiments is consistent with the
picture that they reach chemical equilibrium at a temperature much higher than
the temperature they freeze out kinetically. Using a multiphase transport model
to study particle production in these collisions, we find that the above result
is due to the constancy of the entropy per particle during the evolution of the
hadronic matter from the chemical to the kinetic freeze-out. We further use a
hadron resonance gas model to illustrate the result from the transport model
study.
