Exploring theoretical uncertainties in the hydrodynamic description of relativistic heavy-ion collisions

TL;DR

This paper investigates the impact of theoretical uncertainties, especially causality constraints and second-order transport coefficients, on hydrodynamic models of relativistic heavy-ion collisions, highlighting robustness in large systems and sensitivity in small ones.

Contribution

It systematically analyzes causality conditions and second-order transport effects in hydrodynamic models, providing insights into their influence on flow observables and the robustness of QGP property extraction.

Findings

Flow observables are robust against causality constraints in large systems.

Second-order gradient terms significantly reduce causal regions.

Small systems exhibit high sensitivity to transport coefficients, complicating analysis.

Abstract

We explore theoretical uncertainties in the hydrodynamic description of relativistic heavy-ion collisions by examining the full non-linear causality conditions and quantifying the second-order transport coefficients' role on flow observables. The causality conditions impose physical constraints on the maximum allowed values of inverse Reynolds numbers during the hydrodynamic evolution. Including additional second-order gradient terms in the Denicol-Niemi-Moln\'{a}r-Rischke (DNMR) theory significantly shrinks the casual regions compared to those in the Israel-Stewart hydrodynamics. For Au+Au collisions, we find the variations of flow observables are small with and without imposing the necessary causality conditions, suggesting a robust extraction of the Quark-Gluon Plasma's transport coefficients in previous model-to-data comparisons. However, sizable sensitivity is present in small p+Au collisions, which poses challenges to study the small systems' collectivity.