Viscous Effects on the Mapping of the Initial to Final State in Heavy Ion Collisions

TL;DR

This study examines how shear and bulk viscosity influence the relationship between initial collision geometry and final flow patterns in heavy ion collisions, revealing complex dependencies and the impact of initial flow contributions.

Contribution

It provides a detailed analysis of viscous effects on flow harmonics and compares viscous hydrodynamics with ideal models, highlighting the importance of initial geometry features.

Findings

Viscosity affects the prediction of flow harmonics from initial eccentricities.

Higher-order flow harmonics depend on complex initial geometry aspects.

Initial flow contributions reduce the predictive accuracy of initial eccentricities.

Abstract

We investigate the correlation between various aspects of the initial geometry of heavy ion collisions at the Relativistic Heavy Ion Collider energies and the final anisotropic flow, using v-USPhydro, a 2+1 event-by-event viscous relativistic hydrodynamical model. We test the extent of which shear and bulk viscosity affect the prediction of the final flow harmonics, $v_n$, from the initial eccentricities, $\varepsilon_n$. We investigate in detail the flow harmonics $v_1$ through $v_5$ where we find that $v_1$, $v_4$, and $v_5$ are dependent on more complicated aspects of the initial geometry that are especially important for the description of peripheral collisions, including a non-linear dependence on eccentricities as well as a dependence on shorter-scale features of the initial density. Furthermore, we compare our results to previous results from NeXSPheRIO, a 3+1 relativistic ideal hydrodynamical model that has a non-zero initial flow contribution, and find that the combined contribution from 3+1 dynamics and non-zero, fluctuating initial flow decreases the predictive ability of the initial eccentricities, in particular for very peripheral collisions, but also disproportionately in central collisions.