Collective dynamics in heavy and light-ion collisions. I. Kinetic theory vs hydrodynamics

High-energy nuclear collisions exhibit collective flow, which emerges as a dynamical response of the quark-gluon plasma (QGP) to the initial state geometry of the collision. Collective flow in heavy-ion collisions is usually described within multistage evolution models, which employ a viscous relativistic hydrodynamic description of the space-time evolution of the QGP. By comparing event-by-event simulations in kinetic theory and viscous hydrodynamics in OO, AuAu and PbPb collisions at energies of the Relativistic Heavy Ion Collider (RHIC) and of LHC, we quantify to what extent a macroscopic hydrodynamic description can accurately describe the development of collective flow and to what extent collective flow in small systems, such as OO, is sensitive to the nonequilibrium evolution of the QGP beyond hydrodynamics.