# Flow in AA and pA as an interplay of fluid-like and non-fluid like excitations

## TL;DR

This paper investigates the microscopic structure of quark-gluon plasma by comparing a kinetic theory model with fluid dynamics and experimental data, revealing the dominance of non-hydrodynamic excitations in small and peripheral collisions.

## Contribution

It introduces a kinetic theory model that combines fluid and particle-like excitations and analyzes their relative importance across different collision systems and energies.

## Key findings

- Non-hydrodynamic excitations dominate in pPb collisions at the LHC.
- Fluid-like excitations are dominant in central nucleus-nucleus collisions.
- Non-hydrodynamic contributions are significant in peripheral nucleus-nucleus collisions.

## Abstract

To study the microscopic structure of quark-gluon plasma, data from hadronic collisions must be confronted with models that go beyond fluid dynamics. Here, we study a simple kinetic theory model that encompasses fluid dynamics but contains also particle-like excitations in a boost invariant setting with no symmetries in the transverse plane and with large initial momentum asymmetries. We determine the relative weight of fluid dynamical and particle like excitations as a function of system size and energy density by comparing kinetic transport to results from the 0th, 1st and 2nd order gradient expansion of viscous fluid dynamics. We then confront this kinetic theory with data on azimuthal flow coefficients over a wide centrality range in PbPb collisions at the LHC, in AuAu collisions at RHIC, and in pPb collisions at the LHC. Evidence is presented that non-hydrodynamic excitations make the dominant contribution to collective flow signals in pPb collisions at the LHC and contribute significantly to flow in peripheral nucleus-nucleus collisions, while fluid-like excitations dominate collectivity in central nucleus-nucleus collisions at collider energies.