Global $\Lambda$ polarization in heavy-ion collisions at energies 2.4--7.7 GeV: Effect of Meson-Field Interaction

TL;DR

This paper models the global Lambda polarization in heavy-ion collisions at energies 2.4 to 7.7 GeV, analyzing the effects of thermal vorticity and meson-field interactions, and compares predictions with experimental data.

Contribution

It introduces a combined analysis of thermal vorticity and meson-field effects on Lambda polarization within a three-fluid model at low collision energies.

Findings

Polarization peaks around 3 GeV collision energy.

Polarization is highly sensitive to the interplay of vorticity and meson-field contributions.

Rapidly increasing polarization from midrapidity to forward/backward rapidities.

Abstract

Based on the three-fluid model, the global $\Lambda$ polarization in Au+Au collisions at 2.4 $\leq\sqrt{s_{NN}}\leq$ 7.7 GeV is calculated, including its rapidity and centrality dependence. Contributions from the thermal vorticity and meson-field term (proposed by Csernai, Kapusta and Welle) to the global polarization are considered. The results are compared with data from recent and ongoing STAR and HADES experiments. It is predicted that the polarization maximum is reached at $\sqrt{s_{NN}}\approx$ 3 GeV, if the measurements are performed with the same acceptance. The value of the polarization is very sensitive to interplay of the aforementioned contributions. In particular, the thermal vorticity results in quite strong increase of the polarization from the midrapidity to forward/backward rapidities, while the meson-field contribution considerably flattens the rapidity dependence. The polarization turns out to be very sensitive to details of the equation of state. While collision dynamics become less equilibrium with decreasing collision energy, the present approach to polarization is based on the assumption of thermal equilibrium. It is found that equilibrium is achieved at the freeze-out stage, but this equilibration takes longer at moderately relativistic energies.