重离子碰撞中强子散射对矢量介子自旋排列测量的影响
The effect of hadronic scatterings on the measurement of vector meson spin alignments in heavy-ion collisions
摘要：
相对论性重离子碰撞中矢量介子和超子的自旋排列被认为是全局极化的信号。STAR实验首次观测到了$\rm Λ$偏振。最近，ALICE合作测量了在$\rm \sqrt{s_{NN}}$ = 2.76 TeV的Pb-Pb碰撞中的横向动量（$p_T$）和$K^*$和$ϕ$自旋排列的碰撞中心依赖性。对于$K^*$，在中间-中心碰撞的低$p_T$区域中观察到大信号，而对于$ϕ$，信号小得多，并且这些尚未被理解。由于矢量介子具有不同的寿命，其衰变产物具有不同的散射截面，它们遭受不同的强子效应。本文利用多相输运模型研究了相对论重离子碰撞中强子相互作用对$K^*$、$ϕ$和$ρ$介子自旋排列的影响。我们发现强子散射导致$ρ$和$K^*$介子的自旋排列矩阵元$ρ_{00}$偏离真实值（对$ρ$影响较大），而对$ϕ$介子的影响可以忽略不计.这种影响取决于运动学接受度：当伪快度（$η$）覆盖度较小时，观测到的$ρ_{00}$值低于真实值，而当$η$覆盖度较大时，观测到的影响很小。因此，我们的研究提供了有价值的信息，以了解矢量介子自旋取向信号中观察到的实验。
英文摘要：
Spin alignments of vector mesons and hyperons in relativistic heavy-ion collisions have been proposed as signals of the global polarization. The STAR experiment first observed the $\rm Λ$ polarization. Recently, the ALICE collaboration measured the transverse momentum ($p_T$) and the collision centrality dependence of $K^*$ and $ϕ$ spin alignments in Pb-Pb collisions at $\rm \sqrt{s_{NN}}$ = 2.76 TeV. A large signal is observed in the low $p_T$ region of mid-central collisions for $K^*$ while the signal is much smaller for $ϕ$, and these have not been understood yet. Since vector mesons have different lifetimes and their decay products have different scattering cross sections, they suffer from different hadronic effects. In this paper, we study the effect of hadronic interactions on the spin alignment of $K^*$, $ϕ$ and $ρ$ mesons in relativistic heavy-ion collisions with a multi-phase transport model. We find that hadronic scatterings lead to a deviation of the observed spin alignment matrix element $ρ_{00}$ away from the true value for $ρ$ and $K^*$ mesons (with a bigger effect on $ρ$) while the effect is negligible for the $ϕ$ meson. The effect depends on the kinematic acceptance: the observed $ρ_{00}$ value is lower than the true value when the pseudorapidity ($η$) coverage is small while there is little effect when the $η$ coverage is big. Our study thus provides valuable information to understand the vector meson spin alignment signals observed in the experiments.
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