Investigation of Nonlinear Collective Dynamics in Relativistic Heavy-Ion Collisions Using A Multi-Phase Transport Model

TL;DR

This study uses the AMPT model to analyze how nonlinear flow coefficients develop through different stages of heavy-ion collisions, revealing their connection to initial geometric configurations and medium response.

Contribution

It provides a detailed microscopic investigation of the origin and evolution of the nonlinear response coefficient $oldsymbol{ ext{chi}}_{4,22}$ in uranium-uranium and gold-gold collisions, highlighting its stability as an initial-state indicator.

Findings

$oldsymbol{ ext{chi}}_{4,22}$ increases during collective expansion.

The ratio of $oldsymbol{ ext{chi}}_{4,22}$ between U+U and Au+Au remains stable across stages.

The ratio isolates intrinsic initial geometric correlations.

Abstract

The nonlinear response coefficient, $\chi_{4,22}$, is a crucial observable for probing the dynamical properties of the quark-gluon plasma (QGP). While traditionally understood as a signature of medium response, recent studies suggest that $\chi_{4,22}$ also encapsulates critical information regarding the intrinsic initial-state configuration of the colliding nuclei. In this study, we utilize A Multi-Phase Transport (AMPT) model to investigate the microscopic origin and stage-by-stage development of $\chi_{4,22}$ in $^{238}$U+$^{238}$U and $^{197}$Au+$^{197}$Au collisions at $\sqrt{s_{\rm NN}} = 200$ GeV. By tracking the flow observables through the partonic cascade, quark coalescence, and hadronic rescattering phases, we map the translation of initial geometric eccentricities into final-state momentum anisotropies. Our results demonstrate that the absolute magnitude of $\chi_{4,22}$ increases continuously during the collective expansion, confirming its nature as a dynamically generated medium response. However, the comparative ratio of this coefficient between the U+U and Au+Au systems is stable across all evolutionary stages within statistical uncertainties. This indicates that the ratio approximately cancels complex evolutionary dynamics to isolate intrinsic geometric correlations present at the initial state. These findings provide compelling theoretical support and crucial insights for recent experimental efforts aiming to extract high-order nuclear structure, such as hexadecapole deformation, using nonlinear flow observables.