Are High Energy Heavy Ion Collisions similar to a Little Bang, or just a very nice Firework?

Abstract

The talk is a brief overview of what we recently learned about excited hadronic matter from heavy ion collisions. The central issue is that the systems produced do exhibit macroscopic behavior, it flows and we start getting some idea about its effective Equation of State (EoS). More specifically, we concentrate on elliptic flow, from SPS to RHIC energies, as well as on particle composition and fluctuations. Note that a pressure and the rate of fluctuation relaxation (discussed at the end) are ultimately a measure of a collision rate in the system we would like to understand.