Radial flow observed at AGS/SPS energies is very strong, with collective velo(:ities of matter reaching about 0.5c for central collisions of the heaviest ions. The lattice-based Equation of State (EOS) is however rather soft, due to the QCD phase transition. We show that both statements are consistent only if proper kinetic-based treatment of the freeze-out is made. In fact chemical and thermal freeze-out happen at quite different conditions, especially at SPS. Event-by-event fluctuations can shed new light on this problem. We also propose new model of "anomalous" ,1/~/~' suppression found for PbPb collisions, related it to prolonged lifetiine of dense matter due the "softest t)oint" of the EOS.
1. THE RADIAL FLOW: AN INTRODUCTION
One of the major goals of heavy ion physics is to learn tile EOS of hot/dense hadronic matter. Although for central collisions of heavy ions at the SPS the energy density of tile order of few GeV/frn a is reached, it remains unknown when and how the matter becomes (locally) equilibrated. We also do not know whether new phase of matter -Quark-Gluon Plasma (QGP) -is actually produced. One well-known strategy addressing these issues relies on (very rare) processes happening at earlier stages, the e/m probes [18] or J/¢/, suppression (see section 5). Both lead to exciting experimental findings, much discussed at this conference.
Another approach (to be mostly discussed in this talk) is based on hadronic observables, well measured now in high-statistics experiments. Although re-scatterings tend to erase most traces of the dense stage, some of them are preserved and accumulated during the expansion: collective flow is one of them. Existing data strongly suggest that the hadronic system does indeed behave as a truly macroscopic one. Rather detailed phenomenology of the so called directed flow was covered here by Ollitrault, so I focus on radial (axially symmetric) flow 2 for central collisions.
Its very existence was widely debated for years, but (although at QM97 we have still witnessed remnants of this debate) it seems to be proven now "beyond a reasonable doubt". The HBT data, Coulomb effects (or any reasonable event generator) show that transverse size at freeze-out significantly exceed that of parent nuclei. An excellent test for existence of the flow is provided by deuterons. The shape of their spectrum, its slope *Based on works done in collaboration with C.M.Hung and D.Teaney. Supported in part by US DOE.
2The longitudinal flow was discussed in multiple hydro-based works. However, due to uncertainty in initial conditions, predictive power of hydrodynamics is rather limited.