Collective dynamics in heavy and light-ion collisions. II. Determining the origin of collective behavior in high-energy collisions

Exploiting the first measurements of the same ion species in OO collisions at the Relativistic Heavy Ion Collider (RHIC) and LHC, we propose an observable to distinguish whether collective behavior builds up through a hydrodynamic expansion of a strongly interacting quark-gluon plasma or few final state rescatterings. Our procedure allows one to disentangle the effects of the initial state geometry and the dynamical response mechanism on anisotropic flow. We validate its ability to discriminate between systems with different interaction rates using results from event-by-event simulations in kinetic theory.