Weak and strong coupling equilibration in nonabelian gauge theories

Abstract

We present a direct comparison studying equilibration through kinetic theory at weak coupling and through holography at strong coupling in the same set-up. The set-up starts with a homogeneous thermal state, which then smoothly transitions through an outof-equilibrium phase to an expanding system undergoing boost-invariant flow. This first apples-to-apples comparison of equilibration provides a benchmark for similar equilibration processes in heavy-ion collisions, where the equilibration mechanism is still under debate. We find that results at weak and strong coupling can be smoothly connected by simple, empirical power-laws for the viscosity, equilibration time and entropy production of the system.

Initial conditions for hydrodynamics from weakly coupled pre-equilibrium evolution

“…The scaled times {0.99, 1.6} can be used to initialize simulations when the transport coefficients differ. Such an approach is supported by the reasonably good scaling properties of the hydrodynamization times and prethermal evolution as function of the coupling constant when expressed in terms of the hydrodynamic variables [16,24].…”

“…The details of the scattering kernel have been discussed in refs. [15,16,24] and are briefly repeated here in the appendix A. We use the isotropic screening approximation from [16] which is leading order accurate for parametrically isotropic systems P L /P T ≈ 1.…”

Flow in AA and pA as an interplay of fluid-like and non-fluid like excitations

“…[52])-described by classical gluodynam-ics at early times [53]-can be smoothly evolved to late times where the system allows for a fluid-dynamic description [54][55][56][57]. In support of its potential phenomenological relevance, this perturbative approach-if supplemented with realistic values of the coupling constantleads to short hydrodynamization timescales [55] comparable to those obtained from non-perturbative strong coupling techniques [58][59][60][61][62] and favored in phenomenological models. This approach has been extended to 2+1 dimensional solutions [63] which form the foundation of tools that can be used to match the out-of-equilibrium initial stage models to a hydrodynamic stage in nucleusnucleus collisions [64,65].…”

Chemical Equilibration in Hadronic Collisions

“…In the following we discuss the time evolution of energy density obtained by momentum integration of the non-equilibrium quark and gluon distribution functions. It was observed in previous works [11,12,13,14], that the parametrically large differences in the equilibration rates for different coupling constants λ can be largely scaled out by measuring time in units of relaxation time…”