如何测试能量损失机制中的路径长度依赖性：导致新的可观察性的分析
How to test path-length dependence in energy loss mechanisms: analysis leading to a new observable
摘要：
高$p_\perp$部分子在穿越QCD介质时会损失能量，这通常可以通过抑制来测量，也可以通过各种能量损失模型来预测。不同的能量损失机制的一个关键测试是能量损失如何依赖于穿过的介质的长度（所谓的路径长度依赖性）。即将到来的实验结果将允许，至少在原则上，第一次，清楚地观察如何能量损失的变化与介质的大小，特别是通过比较已经可用的$Pb+Pb$测量与现在即将到来的$Xe+Xe$数据在大型强子对撞机。然而，在实践中，实际上执行这样的测试，它成为至关重要的选择一个最佳的观察。关于这一点，观察到的两个系统的抑制比似乎是一个自然的（和经常提到的）选择。然而，我们表明，提取的路径长度依赖从这个观察将是不可能的。我们在这里提供了一个简单的缩放参数的基础上的分析推导，以及详细的数值计算的基础上，我们先进的能量损失框架，表明不同的观察是适合于此目的。我们称之为可观察的路径长度敏感的抑制比（$R_L^{AB}$），并提供我们的预测实验数据之前变得可用。这一预测也清楚地表明，这一可观察性将允许将相关的理论模型与实验数据进行简单的比较，从而区分不同的（潜在的）能量损失机制，这对于理解所创建的QCD介质的性质至关重要。
英文摘要：
When traversing QCD medium, high $p_\perp$ partons lose energy, which is typically measured by suppression, and also predicted by various energy loss models. A crucial test of different energy loss mechanisms is how the energy loss depends on the length of traversed medium (so-called path-length dependence). The upcoming experimental results will allow to, at least in principle, for the first time, clearly observe how the energy loss changes with the size of the medium, in particular, by comparing already available $Pb+Pb$ measurements with now upcoming $Xe+Xe$ data at the LHC. However, in practice, to actually perform such test, it becomes crucial to chose an optimal observable. With respect to this, a ratio of observed suppression for the two systems may seem a natural (and frequently mentioned) choice. We, however, show that extracting the path-length dependence from this observable would not be possible. We here provide an analytical derivation based on simple scaling arguments, as well as detailed numerical calculations based on our advanced energy loss framework, showing that a different observable is suitable for this purpose. We call this observable path-length sensitive suppression ratio ($R_L^{AB}$) and provide our predictions before experimental data become available. This predictions also clearly show that this observable will allow a simple comparison of the related theoretical models with the experimental data, and consequently to distinguish between different (underlying) energy loss mechanisms, which is in turn crucial for understanding properties of created QCD medium.
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