Limiting attractors in heavy-ion collisions

TL;DR

This paper explores universal features of the early stages of heavy-ion collisions using QCD kinetic theory, introducing the concept of limiting attractors at different couplings to describe pre-hydrodynamic evolution.

Contribution

It introduces the novel concept of limiting attractors at weak and strong couplings, providing a unified description of pre-hydrodynamic evolution in heavy-ion collisions.

Findings

Hydrodynamic limiting attractor governed by viscosity emerges at strong couplings.

A bottom-up limiting attractor at weak couplings describes late-stage thermalization.

Universal parametrizations for jet and heavy-quark momentum broadening ratios are provided.

Abstract

We study universal features of the hydrodynamization process in heavy-ion collisions using QCD kinetic theory simulations for a wide range of couplings. We introduce the new concept of limiting attractors, which are obtained by extrapolation to vanishing and strong couplings. While the hydrodynamic limiting attractor emerges at strong couplings and is governed by the viscosity-related relaxation time scale $\tau_R$, we identify a bottom-up limiting attractor at weak couplings. It corresponds to the late stages of the perturbative bottom-up thermalization scenario and exhibits isotropization on the time scale $\tau_{\text{BMSS}} = \alpha_s^{-13/5}/Q_s$. In contrast to hydrodynamic limiting attractors, at finite couplings the bottom-up limiting attractor provides a good universal description of the pre-hydrodynamic evolution of jet and heavy-quark momentum broadening ratios $\hat q^{yy} / \hat q^{zz}$ and $\kappa_T/\kappa_z$. We also provide parametrizations for these ratios for phenomenological studies of pre-equilibrium effects on jets and heavy quarks.