Traversing the QCD Phase Transition: Quenching Out of Equilibrium vs. Slowing Out of Equilibrium vs. Bubbling Out of Equilibrium

Abstract

I review arguments for the existence of a critical point E in the QCD phase diagram as a function of temperature T and baryon chemical potential \mu. I describe how heavy ion collision experiments at the SPS and RHIC can discover the tell-tale signatures of such a critical point, thus mapping this region of the QCD phase diagram. I contrast the different ways in which the matter produced in a heavy ion collision can be driven out of equilibrium: quenching out of equilibrium (possible, but not guaranteed, if the transition region is traversed at \mu << \mu_E) vs. slowing out of equilibrium (guaranteed for \mu \sim \mu_E) vs. bubbling out of equilibrium (possible, but not guaranteed, for \mu >> \mu_E). Quenching or bubbling create and amplify distinct, detectable, non-gaussian fluctuations. In contrast, slowing out of equilibrium reduces the magnitude of the specific, detectable, gaussian fluctuations which signal the presence of the critical point.