Limits of thermalization in relativistic heavy ion collisions

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, root S-NN, 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 root S-NN 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. (C) 2022 The Author(s). Published by Elsevier B.V.