Spin alignment of vector mesons in heavy-ion collision

We give a brief review on the spin alignment induced by the strong field and the shear-stress tensor. In experiments, a significant positive deviation from 1/3 is observed for the $\phi$ meson, which can be explained by the anisotropy of the strong field fluctuation in the meson’s rest frame, while the anisotropy is mainly a consequence of the motion of meson relative to the quark-gluon plasma. On the other hand, the shear-induced spin alignment is of the order ${10}^{-4}\sim {10}^{-5}$ if the magnitude of thermal shear tensor is ${10}^{-2}$.