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arXiv 2012.03042hep-phnucl-th
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相对论性重离子碰撞初始阶段的能量动量张量

The energy-momentum tensor at the earliest stage of relativistic heavy ion collisions

Margaret E. Carrington, Alina Czajka, Stanislaw Mrowczynski

备注：
41 pages, 6 figures, published version
发表信息：
Eur. Phys. J. A58, 5 (2022)
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摘要：

高能核碰撞产生一个胶子场，可以用彩色玻璃凝聚体（CGC）有效理论在适当的时间$τ\lesssim 1$ fm/c来描述。该理论可以用来计算胶子能量动量张量，它提供了关于色电和色磁场、能量密度、纵向和横向压力以及其他量的早期演化的信息。我们得到了一个解析表达式的能量动量张量使用适当的时间展开，并工作到六阶。计算在技术上是困难的，部分是因为所涉及的术语的数量随着$τ$展开的阶数而迅速增长，但也因为与事件平均化因子的定义、选择调节这些因子的方法以及结果对正则化和核密度分布函数引入的参数的依赖性有关的几个微妙问题。所有这些问题都与一个重要问题密切相关，即我们期望CGC方法能够在多大程度上准确描述重离子碰撞的早期阶段。我们提出的能量密度和纵向和横向压力的演变的一些结果。我们表明，我们的计算给出了物理上有意义的结果，适当的时间值接近水动力模拟初始化的制度。在另一篇论文[1]中，我们详细分析了其他几个可以从能量动量张量计算出来的实验相关量。

英文摘要：

Nuclear collisions at high energies produce a gluon field that can be described using the Colour Glass Condensate (CGC) effective theory at proper times $τ\lesssim 1$ fm/c. The theory can be used to calculate the gluon energy-momentum tensor, which provides information about the early time evolution of the chromo-electric and chromo-magnetic fields, energy density, longitudinal and transverse pressures, and other quantities. We obtain an analytic expression for the energy-momentum tensor using an expansion in the proper time, and working to sixth order. The calculation is technically difficult, in part because the number of terms involved grows rapidly with the order of the $τ$ expansion, but also because of several subtle issues related to the definition of event-averaged correlators, the method chosen to regulate these correlators, and the dependence of results on the parameters introduced by the regularization and nuclear density profile functions. All of these issues are crucially related to the important question of the extent to which we expect a CGC approach to be able to accurately describe the early stages of a heavy ion collision. We present some results for the evolution of the energy density and the longitudinal and transverse pressures. We show that our calculation gives physically meaningful results up to values of the proper time which are close to the regime at which hydrodynamic simulations are initialized. In a companion paper [1] we give a detailed analysis of several other experimentally relevant quantities that can be calculated from the energy-momentum tensor.

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