Study of Lambda polarization at RHIC BES and LHC energies

TL;DR

This paper extends hydrodynamic calculations of Lambda hyperon polarization to RHIC and LHC energies, analyzing the polarization components and their energy dependence in heavy ion collisions.

Contribution

It provides new predictions of Lambda polarization at higher energies using an extended UrQMD+vHLLE model, including the beam direction component.

Findings

Polarization decreases with collision energy.

The beam direction component dominates at high energies.

Predicted polarization values vary from a few percent to permille levels.

Abstract

In hydrodynamic approach to relativistic heavy ion collisions, hadrons with nonzero spin, produced out of the hydrodynamic medium, can acquire polarization via spin-vorticity thermodynamic coupling mechanism. The hydrodynamical quantity steering the polarization is the thermal vorticity, that is minus the antisymmetric part of the gradient of four-temperature field. Based on this mechanism there have been several calculations in hydrodynamic and non-hydrodynamic models for non-central heavy ion collisions in the RHIC Beam Energy Scan energy range, showing that the amount of polarization of produced $\Lambda$ hyperons ranges from few percents to few permille, and decreases with collision energy. We report on an extension of our existing calculation of global $\Lambda$ polarization in UrQMD+vHLLE model to full RHIC and LHC energies, and discuss the component of polarization along the beam direction, which is the dominant one at high energies.