Jozsó's Legacy: Chemical and kinetic freeze-out in heavy-ion collisions

Abstract

We review J. Zimányi's key contributions to the theoretical understanding of dynamical freeze-out in nuclear collisions and their subsequent applications to ultra-relativistic heavy-ion collisions, leading to the discovery of a freeze-out hierarchy where chemical freeze-out of hadron yields precedes the thermal decoupling of their momentum spectra. Following Zimányi's lines of reasoning we show that kinetic freeze-out necessarily leads to a dependence of the corresponding freeze-out temperature on collision centrality. This centrality dependence can be predicted within hydrodynamic models, and for Au+Au collisions at RHIC this prediction is shown to reproduce the experimentally observed centrality dependence of the thermal decoupling temperature, extracted from hadron momentum spectra. The fact that no such centrality dependence is observed for the chemical decoupling temperature, extracted from the hadron yields measured in these collisions, excludes a similar kinetic interpretation of the chemical decoupling process. We argue that the chemical decoupling data from Au+Au collisions at RHIC can only be consistently understood if the chemical freeze-out process is driven by a phase transition, and that the measured chemical decoupling temperature therefore measures the critical temperature of the quark-hadron phase transition. We propose additional experiments to further test this interpretation.

Early Collective Expansion: Relativistic Hydrodynamics and the Transport Properties of QCD Matter

“…Since the fireballs created in the Little Bangs expand about 10 18 times faster than the early universe did at similar temperatures, the time separation between chemical and thermal decoupling shrinks to a few fm/c in heavy-ion collisions. That the two decoupling processes do not happen simultaneously but hierarchically is easily seen from the kinetic decoupling criterium: [29][30][31][32][101][102][103][104][105]…”

“…at times τ > τ 0 , if hydrodynamics is initialized too early [105]; since the expansion rate diverges like 1/τ at early times, hydrodynamics cannot be started until the longitudinal expansion rate has dropped enough to allow for local thermal equilibrium.) Numerical studies [101][102][103][104][105] show that, except near the transverse edge of the fireball where the expansion rate changes rapidly with position, the kinetic freeze-out surfaces defined by Eq. (44) can be well approximated by surfaces of constant temperature.…”

“…This dependence is consistent with hydrodynamic predictions and Eq. (44) [103]: Larger collision systems created in more central collisions Chemical and thermal freeze-out points extracted from heavy-ion collisions at the GSI SIS, BNL AGS, CERN SPS and RHIC. The shaded area indicates the likely location of the quark-hadron phase transition as extracted from lattice QCD and theoretical models.…”

“…Chemical freeze-out at RHIC can therefore not be driven by a local competition between inelastic hadron scattering and hydrodynamic expansion, as described by Eq. (44) [103]. The observed universality of the measured chemical freeze-out temperature and the proximity of the value extracted from experiment to the critical temperature T c of the quark-hadron phase transition predicted by lattice QCD [7][8][9] can only be understood if one assumes that the phase transition itself controls the chemical freeze-out process.…”

Thermal nature of charmonium transverse momentum spectra from Au-Au collisions at the highest energies available at the BNL Relativistic Heavy Ion Collider (RHIC)

“…This experimental observation also supports the coincidence of kinetic and chemical freeze-out for these particle species. Note that the insensitivity of the chemical freeze-out temperature to centrality is natural [29] if chemical freeze-out happens at the phase boundary [30] (see also the recent review article [31]).…”