Hydrodynamic models for heavy-ion collisions and beyond

A generic property of a first-order phase transition in equilibrium, and in the limit of large entropy per unit of conserved charge, is the smallness of the isentropic speed of sound in the "mixed phase". A specific predictio n is that this should lead to a non-isotropic momentum distribution of nucl eons in the reaction plane (for energies similar to 40 A GeV in our model c alculation). On the other hand, we show that from present effective theorie s for low-energy QCD one does not expect the thermal transition rate betwee n various states of the effective potential to be much larger than the expa nsion rate, questioning the applicability of the idealized Maxwell/Gibbs co nstruction. Experimental data could soon provide essential information on t he dynamics of the phase transition.