The measurement of spin polarization and spin alignment of produced hadrons in heavy-ion collisions opens a new avenue to explore the properties of hot and dense QCD matter. We study the spin polarization generated by the hydrodynamic gradients. In addition to the widely studied thermal vorticity effects, we identify an undiscovered contribution from the fluid shear. This shear-induced polarization (SIP) can be viewed as the fluid analog of strain-induced polarization observed in elastic and nematic materials. We obtain the explicit expression for SIP using the quantum kinetic equation and linear response theory. Based on a realistic hydrodynamic, we study the azimuthal angle dependence of spin polarization in heavy-ion collisions. We find that SIP contribution always shows the same azimuthal angle dependence as experimental data and competes with thermal vorticity effects. In the scenario that Λ inherits and memorizes the spin polarization of strange quark, SIP wins the competition, and the resulting azimuthal angle-dependent spin polarizations agree qualitatively with the experimental data.
Refs.
[1] Baochi Fu, Shuai Y.F. Liu, Longgang Pang, Huichao Song and Yi Yin, arXiv: 2103.10403
