Shear-Induced Spin Polarization in Heavy-Ion Collisions

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 model, we compute the differential spin polarization along both the beam direction (z) over cap and the out-plane direction (y) over cap in noncentral heavy-ion collisions at root s(NN) = 200 GeV, including both SIP and thermal vorticity effects. 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 Lambda inherits and memorizes the spin polarization of a strange quark, SIP wins the competition, and the resulting azimuthal angle dependent spin polarization P (y) and P- z agree qualitatively with the experimental data.

Automated summary

The study reveals that in noncentral heavy-ion collisions, the shear-induced polarization (SIP) competes with thermal vorticity effects, with SIP always showing the same azimuthal angle dependence as experimental data. In the scenario where Lambda inherits and memorizes the spin polarization of a strange quark, SIP wins the competition, leading to qualitatively agreement between the resulting azimuthal angle dependent spin polarizations P(y) and P-z and the experimental data.