QCD thermalization: Ab initio approaches and interdisciplinary connections

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. Outstanding problems in QCD include how the strongly correlated quark-gluon matter forms in a heavy-ion collision, its properties off equilibrium, and the thermalization process in the plasma. The theoretical progress in this field in weak-coupling QCD effective field theories and in strong-coupling holographic approaches based on gauge-gravity duality is reviewed. The interdisciplinary connections of different stages of the thermalization process to nonequilibrium dynamics in other systems across energy scales ranging from inflationary cosmology to strong-field QED to ultracold atomic gases are outlined, with emphasis placed on the universal dynamics of nonthermal and hydrodynamic attractors. Measurements in heavy-ion collisions are surveyed that are sensitive to the early nonequilibrium stages of the collision and the potential for future measurements is discussed. The current state of the art in thermalization studies is summarized and promising avenues for further progress are identified.

Automated summary

Heavy-ion collisions at BNL's RHIC and CERN's LHC provide evidence for the formation of quark-gluon plasma, with ongoing research into how this strongly correlated matter forms, its properties in non-equilibrium states, and the thermalization process. Theoretical progress in weak-coupling QCD and strong-coupling holographic approaches is reviewed, along with interdisciplinary connections to non-equilibrium dynamics in various systems. Current measurements in heavy-ion collisions focus on early non-equilibrium stages, with promising avenues for further progress in thermalization studies identified.