标题： 相对论重离子碰撞中的热平衡和化学平衡 显示英文标题

标题： Thermal and Chemical Equilibration in Relativistic Heavy Ion Collisions

摘要： 我们研究了在LHC和RHIC能量下由Au+Au碰撞产生的部分子等离子体的热化和化学平衡，从粒子动量分布在中心区域首次由于纵向冷却变得各向同性的早期时刻开始。 使用玻尔兹曼方程中胶子和夸克碰撞项的弛豫时间近似，以及从最简单的QCD相互作用构建的真实碰撞项，我们表明碰撞时间的行为适合于平衡。 夸克（反夸克）的碰撞时间在整个等离子体寿命中始终大于胶子的碰撞时间，因此胶子在化学和动力学上都比夸克更快地达到平衡。 也就是说，正如Shuryak几年前指出的那样，我们有一个两阶段的平衡场景。 然而，完全的动力学平衡是缓慢的，化学平衡在假设的禁闭相变开始前无法完成，该相变假定发生在$T_c=200$ MeV。 通过比较不同过程的碰撞熵密度速率，我们明确地表明，非弹性过程以及通常认为的\emph{不}弹性过程在等离子体的平衡中占主导地位，并且胶子分支在熵生成中领先于其他过程。 我们还表明，在微扰QCD中，具有更高幂次的\alpha _s 的过程不一定比具有较低幂次的过程在平衡目的上不重要。 系统的平衡状态也起着重要作用。 我们将我们的结果与部分子级联模型的结果进行了比较。 显示英文摘要

摘要： We investigate the thermalization and the chemical equilibration of a parton plasma created from Au+Au collision at LHC and RHIC energies starting from the early moment when the particle momentum distributions in the central region become for the first time isotropic due to longitudinal cooling. Using the relaxation time approximation for the collision terms in the Boltzmann equations for gluons and for quarks and the real collision terms constructed from the simplest QCD interactions, we show that the collision times have the right behaviour for equilibration. The magnitude of the quark (antiquark) collision time remains bigger than the gluon collision time throughout the lifetime of the plasma so that gluons are equilibrating faster than quarks both chemically and kinetically. That is we have a two-stage equilibration scenario as has been pointed out already by Shuryak sometimes ago. Full kinetic equilibration is however slow and chemical equilibration cannot be completed before the onset of the deconfinement phase transition assumed to be at $T_c=200$ MeV. By comparing the collision entropy density rates of the different processes, we show explicitly that inelastic processes, and \emph{not} elastic processes as is commonly assumed, are dominant in the equilibration of the plasma and that gluon branching leads the other processes in entropy generation. We also show that, within perturbative QCD, processes with higher power in \alpha_s need not be less important for the purpose of equilibration than those with lower power. The state of equilibration of the system has also a role to play. We compare our results with those of the parton cascade model.