Directed flow at mid-rapidity in event-by-event hydrodynamics

TL;DR

This paper investigates a newly identified type of directed flow at mid-rapidity in heavy-ion collisions, using event-by-event hydrodynamics to match experimental data and explore initial geometry fluctuations.

Contribution

It introduces the computation of the new mid-rapidity directed flow (v_1) in hydrodynamic simulations, linking it to initial dipole asymmetries and experimental observations.

Findings

The event plane of v_1 correlates with the initial dipole angle.

v_1 is uncorrelated with the reaction plane.

Results agree with STAR experimental data.

Abstract

Fluctuations in the initial geometry of a nucleus-nucleus collision have been recently shown to result in a new type of directed flow (v_1) that, unlike the usual directed flow, is also present at midrapidity. We compute this new v_1 versus transverse momentum and centrality for Au-Au collisions at RHIC using the hydrodynamic code NeXSPheRIO. We find that the event plane of v_1 is correlated with the angle of the initial dipole of the distribution, as predicted, though with a large dispersion. It is uncorrelated with the reaction plane. Our results are in excellent agreement with results inferred from STAR correlation data.