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The QCD Phase Transition: From the Microscopic Mechanism to Signals Talk at Renconres de Moriond - 97

This talk consists of two very different parts: the first one deals with non-perturbative QCD and physics of the chiral restoration, the second with rather low-key (and still unfinished) work aiming at obtaining EOS and other properties of hot/dense hadronic matter from data on heavy ion collisions. The microscopic mechanism for chiral restoration phase transition is a transition from randomly placed tunneling events (instantons) at low T to a set of strongly correlated tunneling-anti-tunneling events (known as instanton - anti-instanton molecules) at high T. Many features of the transition can be explained in this simple picture, especially the critical line and its dependence on quark masses. This scenario predicts qualitative change of the basic quark-quark interactions around the phase transition line, with some states (such as pion-sigma ones) probably surviving even at $T>T_c$. In the second half of the talk we discuss experimental data on collective flow in heavy ion collision, its hydro-based description and relation to equation of state (EOS). A distinct feature of the QCD phase transition region is high degree of ``softness'', (small ratio pressure/energy density). We present some preliminary results indicated that it is indeed needed to explain the radial flow at SPS energies.