Unshadowing the constituent quark number scaling of harmonic flow in heavy-ion collisions

TL;DR

This paper investigates how spectator shadowing affects the constituent quark number scaling of harmonic flow in heavy-ion collisions, providing a method to disentangle shadowing effects from the flow signal to better interpret experimental data.

Contribution

The study introduces Fourier coefficients to quantify azimuthal absorption and demonstrates how shadowing impacts quark number scaling using a Glauber-based toy model.

Findings

Shadowing significantly alters the harmonic flow signal.

Fourier coefficients effectively measure azimuthal absorption.

Results help interpret STAR and FAIR experimental data.

Abstract

Constituent quark number scaling of elliptic flow has been proposed as one key observable to identify the phase transition or the absence of the Quark-Gluon Plasma (QGP) in heavy-ion collisions. At the fixed target program at RHIC the STAR collaboration has recently reported that NCQ scaling breaks when decreasing the collision energy from $\sqrt{s_\mathrm{NN}} = 4.5$ to $3.0$ GeV. However, the generation of elliptic flow is dominated by a highly intricate interplay of spectator shadowing, squeeze-out and geometry dependent hadron emission governed by their cross sections. Therefore in this article we will disentangle the shadowing contribution from the harmonic flow signal of the particle emitting source, effectively ``unshadowing'' the source. We introduce Fourier coefficients that quantify the azimuthal absorption rate of hadrons decoupling from the system. We benchmark the derived results using a toy model based on a ballistic Glauber description of the penetrating nuclei and calculate how shadowing qualitatively alters the constituent quark number scaling of the hadron emitting source. The results are thus relevant for interpreting recent STAR measurements as well as the upcoming measurements by CBM at FAIR.