Shear-Induced Spin Polarization in Heavy-Ion Collisions

• Published in: Physical review letters 2021-10, Vol.127 (14), p.1-142301, Article 142301
• Main Authors: Fu, Baochi, Liu, Shuai Y. F., Pang, Longgang, Song, Huichao, Yin, Yi
• Format: Article
• Language: English
• Subjects: Electrons; Heavy ions; Induced polarization; Ionic collisions; Kinetic equations; Polarization (spin alignment); Shear; Strain; Vorticity
• ISSN: 0031-9007; 1079-7114
• DOI: 10.1103/PhysRevLett.127.142301

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 and the out-plane direction y in noncentral heavy-ion collisions at √sNN = 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 Λ inherits and memorizes the spin polarization of a strange quark, SIP wins the competition, and the resulting azimuthal angle dependent spin polarization Py and Pz agree qualitatively with the experimental data.