各向异性古布瑟流中的局域自旋极化：抑制机制与公式依赖性

中文摘要 AI 辅助

我们利用扰动古布瑟（Gubser）流解析解，对相对论重离子碰撞中的纵向自旋极化开展解析研究。在大系统尺寸极限下，我们推导得到了沿束流方向的局域自旋极化解析表达式。在我们的处理中，热涡度与热剪切的贡献量级相当；热涡度产生的极化符号与实验观测结果相反，而热剪切则抵消该效应，助力恢复所需符号。我们发现，选择与流体速度对齐的参考单位矢量时，仅在低横动量下能得到实验观测到的符号；而采用沿实验室时间方向固定单位矢量的另一公式，能在宽横动量范围内得到所需符号。值得注意的是，近期提出的、单位矢量沿冻出超曲面法向的公式，在大系统尺寸极限的领头阶下，热涡度与热剪切的贡献存在精确抵消。我们识别出一种以加速度主导的普遍抵消模式，该模式在后两种公式中尤为明显。因此，总极化源于非加速度效应，即便椭圆流有限，该效应也未必显著，这在我们的解析结果中得到了清晰体现。作为对比，我们还讨论了带转动的哈勃（Hubble）流中的自旋极化。

英文摘要

We analytically study the longitudinal spin polarization in relativistic heavy-ion collisions using a perturbed Gubser flow solution. In the large-system-size limit, we derive analytical expression of the local spin polarization along the beam direction. In our treatment, the contributions from thermal vorticity and thermal shear are of comparable magnitudes. The thermal vorticity yields the polarization with a sign opposite to that observed experimentally, while the thermal shear counteracts this effect, helping recover the desired sign. We find that the choice of the reference unit vector aligned with the fluid velocity gives the experimentally observed sign only at low transverse momenta, whereas another formulation with the unit vector fixed along the laboratory time direction yields the desired sign for a wide range of transverse momenta. Notably, a recent formulation with the unit vector normal to the freeze-out hypersurface exhibits an exact cancellation between contributions from thermal vorticity and thermal shear at leading order in the large-system-size limit. We identify a general cancellation pattern with acceleration dominance, which is manifest particularly in the latter two formulations. Thus, the total polarization originates from non-acceleration effects, which need not be substantial even when the elliptic flow is finite, as clearly demonstrated in our analytical results. For comparison, we also discuss the spin polarization in the Hubble flow with rotation.

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13 pages, 5 figures