Beyond the hydrodynamics horizon in the evolution of small and large colliding systems at colliders

A hot expanding environment is produced in heavy-ion (e.g., lead-lead or gold-gold) collisions in the Large Hadron Collider (LHC) and Relativistic Heavy Ion Collider (RHIC). The produced system expands and cools down, turning from a phase with liberated quarks and gluons, called quark-gluon plasma (QGP), to hadrons, detectable in the detectors. The QGP behaves like a nearly perfect fluid that can be modeled via relativistic hydrodynamics with the smallest observed shear and bulk viscosity over entropy density. In the course of the collective expansion, the degrees-of-freedom interaction develops correlation, reflected in the correlation among final hadrons. Models based on hydrodynamics successfully describe the observed correlations in the experiments. Observing a similar correlation among final hadrons emitted from much smaller collision systems, e.g., proton-proton and proton-lead, has triggered debates about the nature of the collectivity in such scenarios. Studies show that the models based on hydrodynamics become less predictive in smaller system collisions. In these systems, one does not expect a thermalized medium, and a framework beyond hydrodynamics is required to explain the true underlying mechanism in collective expansion. The main objective of the current project is to prepare a computational tool in the form of an event generator based on the kinetic theory with isotropization time approximation. Among an extensive list of heavy-ion collective models, this event generator will be unique in explaining small systems that behave particle-like and large systems that act fluid-like in a single framework. The model can bridge the experimental measurements and theoretical studies to quantitatively analyze the fluid-like/particle-like nature of large and small system collisions.