Limits of thermalization in relativistic heavy ion collisions

Abstract

The goal of collider programs of high energy heavy-ion collisions is to produce a form of matter that can test non-perturbative aspects of Quantum Chromodynamics (QCD), the theory of strong interactions. One of the primary predictions from a computational approach to this quantum field theory is the existence of distinct thermodynamic states of QCD, particularly a state of deconfined quarks and gluons. An important step in establishing changes of state of QCD matter is to demonstrate that femto-scale system created in the laboratory can attain thermodynamic equilibrium. We present such a demonstration by checking that event-to-event fluctuations of several different conserved quantities have a consistent treatment in a grand canonical ensemble through a common temperature and chemical potentials. A key component of this work is that we used cumulants of up to third order in the net proton number, charge, and strangeness. We found a clear indication that the femto-scale fireball is thermalized when the collision energy in the centre of mass, sNN, is high enough. Also, the use of higher order cumulants, within the framework of grand canonical ensemble, reveals for the first time that the fireballs created at lower sNN are not in thermodynamic equilibrium. This opens the door to further studies of the approach to equilibrium, and the effects that a critical point in the phase diagram of QCD could have on such phenomena.