The quark-gluon plasma produced by collisions between ultra-relativistic heavy nuclei is well described in the language of hydrodynamics. Non-central collisions are characterized by very large angular momentum, which in a fluid system manifests as flow vorticity. This rotational structure can lead to a spin polarization of the hadrons that eventually emerge from the plasma, providing experimental access to flow substructure at unprecedented detail. Recently, first observations of $\Lambda$ hyperon polarization along the direction of collisional angular momentum have been reported. These measurements are in broad agreement with hydrodynamic and transport-based calculations and reveal that the QGP is the most vortical fluid ever observed. However, there remain important tensions between theory and observation which might be fundamental in nature. In the relatively mature field of heavy ion physics, the discovery of global hyperon polarization and three-dimensional simulations of the collision have opened an entirely new direction of research. We discuss the current status of this rapidly developing area and directions for future research.

中文翻译：

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夸克-胶子等离子体中的极化和涡度

超相对论重核之间碰撞产生的夸克-胶子等离子体用流体动力学的语言进行了很好的描述。非中心碰撞的特点是非常大的角动量，在流体系统中表现为流动涡度。这种旋转结构可以导致最终从等离子体中出现的强子的自旋极化，从而以前所未有的细节提供对流子结构的实验访问。最近，已经报道了沿碰撞角动量方向的 $\Lambda$ 超子极化的首次观察。这些测量结果与基于流体动力学和传输的计算大体一致，并表明 QGP 是有史以来观察到的涡旋最大的流体。然而，理论和观察之间仍然存在重要的紧张关系，这可能是本质上的根本。在相对成熟的重离子物理领域，全球超子极化的发现和碰撞的三维模拟开辟了一个全新的研究方向。我们讨论了这个快速发展领域的现状和未来研究的方向。