Directed flow at midrapidity in heavy-ion collisions

TL;DR

This paper discusses the observation and measurement of directed flow asymmetry at midrapidity in heavy-ion collisions, highlighting its significance in understanding initial geometry fluctuations and long-range correlations.

Contribution

It introduces a new measurement method for directed flow at midrapidity and provides evidence linking initial geometry fluctuations to observed asymmetries.

Findings

Evidence of dipole asymmetry at midrapidity from STAR data

Extraction of differential directed flow from experimental data

Proposal of a new direct measurement technique

Abstract

It was recently shown that fluctuations in the initial geometry of a heavy ion collision generally result in a dipole asymmetry of the distribution of outgoing particles. This asymmetry, unlike the usual directed flow, is expected to be present at a wide range of rapidity -- including midrapidity. First evidence of this phenomenon can be seen in recent two-particle correlation data by STAR, providing the last element necessary to quantitatively describe long-range dihadron correlations. We extract differential directed flow from these data and propose a new direct measurement.