标题： 高能核碰撞中的膨胀、热化和熵产生 显示英文标题

标题： Expansion, Thermalization and Entropy Production in High-Energy Nuclear Collisions

摘要： 在膨胀的夸克胶子气体中研究了热化过程，使用包含两种碰撞项的玻尔兹曼方程。在弛豫时间近似下，我们确定了时间依赖的弛豫时间导致夸克热化的条件。我们计算了由于碰撞引起的熵产生，针对一般的时间依赖弛豫时间。另一方面，在微扰QCD方法中，我们可以估计夸克碰撞时间和其随膨胀时间的变化。有效“非平衡”碰撞时间与标准输运弛豫时间$\tau_{\rm tr}\simeq(\alpha_s^2\ln(1/\alpha_s)T)^{-1}$相比，具有较弱的时间依赖性。在两种情况下，德拜屏蔽和朗道阻尼调节了奇异的前向散射过程。我们发现夸克气体最终会热化，但仅在经历了自由传播和逐渐平衡的阶段后，期间产生了大量熵（“后续燃烧”）。最终的熵以及因此粒子密度取决于碰撞时间和初始条件（“记忆效应”）。基于早期夸克产生的各种模型估计，给出了熵产生的结果。 显示英文摘要

摘要： The thermalization process is studied in an expanding parton gas using the Boltzmann equation with two types of collision terms. In the relaxation time approximation we determine the criteria under which a time-dependent relaxation time leads to thermalization of the partons. We calculate the entropy production due to collisions for the general time-dependent relaxation time. In a perturbative QCD approach on the other hand, we can estimate the parton collision time and its dependence on expansion time. The effective `out of equilibrium' collision time differs from the standard transport relaxation time, $\tau_{\rm tr}\simeq(\alpha_s^2\ln(1/\alpha_s)T)^{-1}$, by a weak time dependence. It is in both cases Debye screening and Landau damping that regulate the singular forward scattering processes. We find that the parton gas does thermalize eventually but only after having undergone a phase of free streaming and gradual equilibration where considerable entropy is produced (``after-burning"). The final entropy and thus particle density depends on the collision time as well as the initial conditions (a ``memory effect"). Results for entropy production are presented based upon various model estimates of early parton production.