QCD thermalization: Ab initio approaches and interdisciplinary connections

TL;DR

This review surveys ab initio approaches to QCD thermalization in ultrarelativistic heavy-ion collisions, contrasting weak-coupling CGC/Glasma descriptions with strong-coupling holographic models. It highlights universal attractor behavior and self-similar scaling that bridge far-from-equilibrium dynamics to hydrodynamics and kinetic theory, connecting early-time evolution to observable bulk flows and entropy production. It also emphasizes interdisciplinary connections to inflationary preheating, ultracold quantum gases, and anomalous transport phenomena, and discusses experimental signatures and future measurements including EIC opportunities. Overall, the work outlines a coherent framework that unifies weak- and strong-coupling pictures, maps complex non-equilibrium QCD dynamics onto tractable effective theories, and points to promising avenues for quantitative, cross-disciplinary progress.

Abstract

Heavy-ion collisions at BNL's Relativistic Heavy Ion Collider and CERN's Large Hadron Collider provide strong evidence for the formation of a quark-gluon plasma, with temperatures extracted from relativistic viscous hydrodynamic simulations shown to be well above the transition temperature from hadron matter. How the strongly correlated quark-gluon matter forms in a heavy-ion collision, its properties off-equilibrium, and the thermalization process in the plasma, are outstanding problems in QCD. We review here the theoretical progress in this field in weak coupling QCD effective field theories and in strong coupling holographic approaches based on gauge-gravity duality. We outline the interdisciplinary connections of different stages of the thermalization process to non-equilibrium dynamics in other systems across energy scales ranging from inflationary cosmology, to strong field QED, to ultracold atomic gases, with emphasis on the universal dynamics of non-thermal and of hydrodynamic attractors. We survey measurements in heavy-ion collisions that are sensitive to the early non-equilibrium stages of the collision and discuss the potential for future measurements. We summarize the current state-of-the art in thermalization studies and identify promising avenues for further progress.

Figures (32)

• Figure 1: Transverse hadron profile resolved in scattering with fixed squared momentum transfer $Q^2$ and increasing center-of-mass energy $\sqrt{s}$. The requirement for proliferating soft gluons to have maximal occupancy $1/\alpha_S$ generates the close packing saturation scale $Q_S$. Adapted from Iancu:2003xm.
• Figure 2: DIS in the dipole picture. The virtual photon emitted by the electron splits into a $q{\bar{q}}$ dipole that scatters off dynamical small $x$ gauge fields coupled to the static large $x$ lightcone sources. From Iancu:2003xm.
• Figure 3: Unintegrated gluon distribution (in units of the transverse area) vs the squared transverse momentum (normalized to its value at the peak of the initial condition curve) from the solution of the Balitsky-Kovchegov equation. The different curves represent increasing rapidities (left to right) for fixed and running coupling. From Dusling:2009ni.
• Figure 4: Solution of the JIMWLK equation for the correlator of Wilson lines $V(x_\perp) V^\dagger(y_\perp)$ probed by the DIS dipole Dumitru:2011vk. As the nucleus is boosted from low energy (or rapidity) to high energy, the regions with large values of these correlator shrink spatially, corresponding to larger values of $Q_S$.
• Figure 5: Top panel: multi-particle production from cut "vacuum-vacuum" graphs connecting time dependent sources of the two nuclei after the collision. From Gelis:2010nm. Bottom panel: the Schwinger-Keldysh closed time contour on which the sources and fields are defined.