Evidence for a phase with high specific entropy in nuclear collisions

We determine the entropy-per-baryon content of the central reaction region in terms of the charged particle multiplicity. We study the consistency of our findings with recent data on strange antibaryon production at 200A GeV in S-A collisons (A∼200) assuming formation of a central fireball. Hadron gas models which do not invoke strong medium modifications of hadron masses do not provide enough entropy and are inconsistent with the combined experimental results. In contrast the quark-gluon plasma hypothesis explains them naturally.