2022-03-04 11:00  P5A-1

Spin polarization of particles in a thermal model

Avdhesh Kumar

In non-central heavy ion collisions at the relativistic beam energies colliding nuclei carry a huge orbital angular momentum. Soon after the collision, a substantial part of such an angular momentum gets deposited in the interaction zone which can further be transformed from purely orbital to the spin form. The latter can be naturally displayed in the spin polarization of the produced particles. Indeed, the spin polarization of various particles (Λ, K*, φ) produced in heavy-ion collisions has been recently measured by the STAR and ALICE collaboration. The results obtained in experiments indicates that the global spin polarization is along the direction perpendicular to the reaction plane, resembling to the magneto-mechanical Barnett effect and Einstein-de Haas effect. Theoretically, relativistic hydrodynamic models that uses the thermal vorticity as the basic quantity responsible for spin polarization are successfully able to explain the experimental measurements of global spin polarization (integrated over all momenta). However, the predictions of hydrodynamic models fails to reproduce the momentum dependence for another experimental observable known as longitudinal spin polarization (spin polarization along the beam direction) of Λ-hyperons. Hydrodynamic model calculations yields quadrupole structure of momentum dependence of longitudinal spin polarization which has an opposite sign compared to that found in the experiment. This mismatch between theoretical and experimental results is known as the sign problem. Recently, it was found that sign problem can be resolved by including the previously overlooked shear-induced polarization governed by a symmetric tensor known as thermal shear tensor. In this seminar, by making use of the direct connection of spin polarization with the vortical properties of the fluid described by thermal vorticity plus an overlooked thermal shear term I will discuss how a simple thermal model with single freeze-out can be used to determine various observables related to Λ-hyperon spin polarization. Our results show that unlike the previous studies done by using only the thermal vorticity, the thermal shear term alone leads to the correct sign of the quadrupole structure of the longitudinal component of the polarization three-vector measured in experiments. However, we find almost complete cancellation between thermal shear and vorticity terms, which eventually leads to disagreement with the data.

Based on References:

1. Effect of thermal shear on longitudinal spin polarization in a thermal model, Wojciech Florkowski, Avdhesh Kumar*, Aleksas Mazeliauskas, and Radoslaw Ryblewski, arXiv:2112.02799v1 [hep-ph].
2. Longitudinal spin polarization in a thermal model, Wojciech Florkowski, Avdhesh Kumar*, Aleksas Mazeliauskas, and Radoslaw Ryblewski, Phys. Rev. C 100, 054907 (2019); arXiv:1904.00002 [nucl-th].

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