Universal chemical freeze-out as a phase transition signature

It is shown that kinetic freeze-out in relativistic heavy-ion collisions invariably entails a non-trivial dependence of the freeze-out temperature on the collision centrality. The centrality independence of the chemical freeze-out temperature observed in Au+Au collisions at RHIC is therefore inconsistent with the hypothesis that hadron abundances decouple kinetically from inelastic hadron-hadron interactions. On the other hand, it is consistent with the hypothesis that chemical decoupling is driven by the quark-hadron phase transition, and that the observed universal chemical freeze-out reflects its critical temperature, independent of the dynamical state of the collision fireball as it passes through the phase transition.