Lawrence Berkeley National Laboratory

In this paper we discuss the fluidity of the hot and dense QCD matter created in ultrarelativistic heavy ion collisions in comparison with various other fluids and, in particular, suggest its possible supercriticality. After examining the proper way to compare nonrelativistic and relativistic fluids from both thermodynamic and hydrodynamic perspectives, we propose a new fluidity measure that shows certain universality for a remarkable diversity of critical fluids. We then demonstrate that a fluid in its supercritical regime has its fluidity considerably enhanced. This result may suggest a possible relationship between the seemingly good fluidity of the QCD matter produced in heavy ion collisions at a center-of-mass energy of s=200A GeV and the supercriticality of this matter with respect to the critical end point on the QCD phase diagram. Based on this observation, we predict an even better fluidity of the matter to be created in heavy ion collisions at Large Hadron Collider energies and the loss of good fluidity at certain, lower beam energy. Finally, based on our criteria, we analyze the suitability of a hydrodynamic description for the fireball evolution in heavy ion collisions at various energies. © 2010 The American Physical Society.