Hydrodynamics from free-streaming to thermalization and back again

Abstract

We study the evolution of the Knudsen and Reynolds numbers in (0+1)-dimensionally expanding fluids with Bjorken symmetry for systems whose microscopic mean free path rises more quickly with time than usually assumed. This allows us to explore within a simple 1-dimensional model the transition from initially thermalizing to ultimately decoupling dynamics. In all cases studied the dynamics is found to be controlled by a hydrodynamic attractor for the Reynolds number whose trajectory as a function of Knudsen number makes a characteristic turn as the dynamics changes from thermalizing to decoupling. We argue that this feature is robust and should also manifest itself in realistic 3-dimensional simulations of expanding heavy-ion collision fireballs.

Nonequilibrium Attractor in High-Temperature QCD Plasmas

“…The temperature T here corresponds to the temperature of an equilibrium system with the same energy density, given by T ¼ ð30ε=νπ 33 , in panels (a), (b), and (c), respectively. These simulations have been initialized with either of the two following initial conditions: (i) spheroidally deformed thermal initial conditions which we will refer to as "RS" initial conditions [44] and (ii) nonthermal color-glass-condenssate (CGC) inspired initial conditions [8].…”

Approach to thermalization and hydrodynamics

“…( 9) in derivatives of f-this is an analog of the slow-roll expansion in theories of inflation (see, e.g., Ref. [21]). At leading order one finds…”

Quarkonia Formation in a Holographic Gravity–Dilaton Background Describing QCD Thermodynamics

“…Since heavy quarks essentially emerge in early, hard processes, they witness the course of heavy-ion collision-either as individual entities or subjects of dissociating and regenerating bound states [3][4][5]. Accordingly, the heavy-quark physics addresses such issues as charm (c,c) and bottom (b,b) dynamics related to transport coefficients [6][7][8][9][10] in the rapidly evolving and highly anisotropic ambient quark-gluon medium [11,12] as well as cc and bb states as open quantum systems [13][14][15][16]. The rich body of experimental data from LHC, and also from RHIC , enabled a tremendous refinement of our understanding of heavy-quark dynamics.…”