We explore theoretical uncertainties in the hydrodynamic description of relativistic heavy-ion collisions by examining the full nonlinear 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-Molnár-Rischke (DNMR) theory significantly shrinks the casual regions compared to those in the Israel-Stewart hydrodynamics. For $\mathrm{Au}+\mathrm{Au}$ 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 $p+\mathrm{Au}$ collisions, which poses challenges to study the small systems' collectivity.

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探索相对论重离子碰撞流体动力学描述中的理论不确定性

我们通过检查完全非线性因果关系条件和量化二阶输运系数对流动可观察量的作用，探索相对论重离子碰撞的流体动力学描述中的理论不确定性。在流体动力学演化过程中，因果关系条件对逆雷诺数的最大允许值施加了物理约束。在 Denicol-Niemi-Molnár-Rischke (DNMR) 理论中包含额外的二阶梯度项，与 Israel-Stewart 流体动力学中的那些相比，可以显着缩小临时区域。为了$金+金$在碰撞中，我们发现在施加和不施加必要的因果关系条件的情况下，流动可观测值的变化很小，这表明在之前的模型到数据的比较中对夸克-胶子等离子体的传输系数进行了稳健的提取。然而，在小的情况下存在相当大的敏感性$磷+金$ 碰撞，这对研究小系统的集体构成了挑战。