Event-by-event fluctuations in the initial stages of ultrarelativistic nucleus-nucleus collisions depend little on rapidity. The hydrodynamic expansion which occurs in later stages then gives rise to correlations among outgoing particles which depend weakly on their relative rapidity. Azimuthal correlations, through which anisotropic flow (vn(pT)) is defined, have been the most studied. Here we study a new observable introduced in 2020 by Schenke, Shen and Teaney and dubbed v0(pT), which quantifies the relative change in the pT spectrum induced by a fluctuation. We describe how it can be measured. Using hydrodynamic simulations, we make quantitative predictions for v0(pT) of charged and identified hadrons. We then discuss how v0(pT) relates to two phenomena which have been measured: The increase of the mean transverse momentum in ultracentral collisions, and the event-by-event fluctuations of the transverse momentum per particle [pT]. We show that v0(pT) determines the dependence of these quantities on the pT cuts implemented in the analysis. We quantitatively explain the rise of σpT observed by ATLAS as the upper pT-cut is increased from 2 to 5 GeV/c.

中文翻译：

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利用 pT 谱波动探测重离子碰撞中的集体


超相对论核-核碰撞初始阶段逐个事件的波动几乎不依赖于速度。随后阶段发生的流体动力膨胀会引起传出粒子之间的相关性，这种相关性弱依赖于它们的相对速度。方位角相关性是研究最多的，通过方位角相关性来定义各向异性流 (vn(pT))。在这里，我们研究了 Schenke、Shen 和 Teaney 在 2020 年引入的一个新的可观测量，称为 v0(pT)，它量化了波动引起的 pT 谱的相对变化。我们描述了如何测量它。使用流体动力学模拟，我们对带电和已识别强子的 v0(pT) 进行定量预测。然后我们讨论 v0(pT) 如何与已测量的两个现象相关：超中心碰撞中平均横向动量的增加，以及每个粒子横向动量的逐事件波动 [pT]。我们证明 v0(pT) 决定了这些量对分析中实施的 pT 切割的依赖性。我们定量地解释了 ATLAS 观察到的 σpT 的上升，因为上 pT 截止从 2 GeV/c 增加到 5 GeV/c。