标题： π介子气体中ρ介子的自旋取向 显示英文标题

标题： The spin alignment of rho mesons in a pion gas

摘要： 我们利用自旋运动方程或玻尔兹曼方程研究了π介子气体中中性ρ介子的自旋取向。 $\rho\pi\pi$耦合由手征有效理论给出。 自旋玻尔兹曼方程中领头阶和次领头阶碰撞项被推导出来。 用不同的初始条件模拟了中性ρ介子的自旋密度矩阵演化。 数值结果显示，如果系统初始时没有ρ介子，π介子与中性ρ介子的相互作用在中心快度区域内产生非常小的自旋取向；这种在中心快度区域内的小自旋取向将在之后迅速衰减至零。如果初始时刻存在具有显著自旋取向的ρ介子，自旋取向也会快速减小。 我们还考虑了爆轰波模型中π介子椭圆流对$\rho_{00}$的影响。 初始时刻自旋取向消失的情况下，$\rho_{00}$与1/3的偏差为正但非常小。 显示英文摘要

摘要： We study the spin alignment of neutral rho mesons in a pion gas using spin kinetic or Boltzmann equations. The $\rho\pi\pi$ coupling is given by the chiral effective theory. The collision terms at the leading and next-to-leading order in spin Boltzmann equations are derived. The evolution of the spin density matrix of the neutral rho meson is simulated with different initial conditions. The numerical results show that the interaction of pions and neutral rho mesons creates very small spin alignment in the central rapidity region if there is no rho meson in the system at the initial time. Such a small spin alignment in the central rapidity region will decay rapidly toward zero in later time. If there are rho mesons with a sizable spin alignment at the initial time the spin alignment will also decrease rapidly. We also considered the effect on $\rho_{00}$ from the elliptic flow of pions in the blast wave model. With vanishing spin alignment at the initial time, the deviation of $\rho_{00}$ from 1/3 is positive but very small.