Shear-Induced Spin Polarization in Heavy-Ion Collisions.

Baochi Fu,Longgang Pang,Yi Yin,Shuai Y F Liu,Huichao Song

doi:10.1103/physrevlett.127.142301

Abstract

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 ^] and the out-plane direction y[over ^] in noncentral heavy-ion collisions at sqrt[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 Λ 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.

Highlights

We study the spin polarization generated by the hydrodynamic gradients
To investigate the generality of the results reported above, we repeat our calculations by systematically varying inputs for the present hydrodynamic model, including initial conditions, freeze-out temperature, and the parametrization of the shear viscosity, bulk viscosity, and equation of state (EoS); see our upcoming publication for more details [71]
Except for some specific cases when a unusually large T gradient is caused by EoSs much harder than the lattice EoS or a sharply peaked bulk viscosity around the freeze-out region, the azimuthal angle dependence of spin polarization is generically dominated by shear-induced polarization (SIP) and shows the qualitative agreement with the data

Summary

Introduction

We study the spin polarization generated by the hydrodynamic gradients. Based on a realistic hydrodynamic model, we compute the differential spin polarization collisions at aploffisnffiffiNgffiffiNffiffiffib1⁄4oth20t0heGebVea, mincdliuredcintigonbozthanSdIPtheanodutt-hpelramneal direction vorticity yin noncentral heavy-ion effects. One widely studied effect is the spin polarization induced by thermal vorticity [37,38,39], a specific combination of temperature gradient and fluid vorticity.

Results

Conclusion