Collective flow without hydrodynamics: simulation results for relativistic ion collisions

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\, d + Au , ³He + Au\, 3 He + Au at √s=200 s = 200 GeV and p + Pb\, p + Pb collisions at √s=5 s = 5 TeV, respectively. For comparison, also Pb + Pb\, Pb + Pb collisions at √s=2.76 s = 2.76 TeV are simulated. It is found that non-hydrodynamic evolution can give rise to radial flow equal to or larger than hydrodynamics with η /s=0.08 η / 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\, Pb + Pb collisions at √s=2.76 s = 2.76 TeV.