Collective flow without hydrodynamics: simulation results for relativistic ion collisions

TL;DR

This study shows that non-hydrodynamic models like free-streaming can produce flow signatures similar to hydrodynamics in relativistic ion collisions, challenging the traditional interpretation of flow as a hydrodynamic effect.

Contribution

It demonstrates that flow signatures in relativistic ion collisions can arise from non-hydrodynamic dynamics, providing an alternative explanation to hydrodynamic models.

Findings

Non-hydrodynamic evolution can produce comparable or larger radial flow.

Free-streaming generates flow patterns similar to hydrodynamics in light-on-heavy collisions.

Elliptic flow from free-streaming is generally smaller than from hydrodynamics in nucleus-nucleus collisions.

Abstract

Flow signatures in experimental data from relativistic ion collisions are usually interpreted as a fingerprint of the presence of a hydrodynamic phase during the evolution of these systems. In this work, flow signatures arising from event-by-event viscous hydrodynamics are compared to those arising from event-by-event non-interacting particle dynamics (free-streaming), both followed by a late-stage hadronic cascade, in d+Au, 3He+Au at sqrt(s)=200 GeV and p+Pb collisions at sqrt(s)=5 TeV, respectively. For comparison, also Pb+Pb collisions at sqrt(s)=2.76 TeV are simulated. It is found that non-hydrodynamic evolution can give rise to equal or larger radial flow than hydrodynamics with eta/s=0.08 in all simulated collision systems. In light-on-heavy-ion collisions, free-streaming gives rise to triangular and quadrupolar flow comparable to or larger than that from hydrodynamics, but it generally leads to considerably smaller elliptic flow. As expected, free-streaming leads to considerably less elliptic, triangular and quadrupolar flow than hydrodynamics in nucleus-nucleus collisions, such as event-by-event Pb+Pb collisions at sqrt(s)=2.76 TeV.