It is frequently supposed that quark-gluon plasma created in heavy-ion collisions undergoes free streaming at early times. We examine this issue based on the assumption that a universal attractor dominates the dynamics already at the earliest stages, which offers a way to connect the initial state with the start of the hydrodynamic expansion in an approximate but conceptually transparent fashion. We demonstrate that the centrality dependence of the measured particle multiplicities can be used to quantitatively constrain the pressure anisotropy and find that it strongly depends on the model of the initial energy deposition. As an illustration, we compare three initial state models and show that they predict rather different early-time values of the pressure anisotropy. This suggests that while assuming free streaming prior to hydrodynamization may be compatible with some initial state models, in general, features of the prehydrodynamic flow need to be matched with the model of the initial state.

中文翻译：

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限制超相对论核碰撞的初始阶段

人们经常假设重离子碰撞中产生的夸克-胶子等离子体在早期经历了自由流动。我们基于通用吸引子在最早阶段已经主导动力学的假设来研究这个问题，这提供了一种以近似但概念透明的方式将初始状态与流体动力学膨胀的开始联系起来的方法。我们证明了测量的粒子多重性的中心性依赖性可用于定量约束压力各向异性，并发现它强烈依赖于初始能量沉积的模型。作为说明，我们比较了三个初始状态模型，并表明它们预测压力各向异性的早期值相当不同。