Helicity and vorticity in heavy-ion collisions at NICA energies

TL;DR

This study investigates the angular momentum transfer, vorticity, and helicity in heavy-ion collisions at NICA energies using the PHSD model, revealing vortex ring formations and helicity separation related to hyperon polarization.

Contribution

It provides detailed analysis of vorticity and helicity fields in heavy-ion collisions, demonstrating vortex ring formation and helicity separation, which are linked to hyperon polarization.

Findings

Formation of oppositely-rotating vortex rings along the z-axis.

Helicity separation correlates with hyperon momentum projections.

Vorticity magnitude is comparable to shear deformation, smaller than Poiseuille flow.

Abstract

Heavy-ion collisions at center-of-mass nucleon collision energies 4.5--11.5 GeV are analyzed within the PHSD transport model. Spectator nucleons are separated, and the transfer of the initial angular momentum of colliding nuclei to the fireball formed by participants is studied. The maximal angular momentum is carried by the fireball in gold-gold collisions with the impact parameter about 5 fm corresponding to centrality class 10--20\%. The obtained participant distributions were fluidized and the energy and baryon number densities, temperature, and velocity fields are obtained in the Landau frame. It is shown that the velocity field has dominantly Hubble-like transversal and longitudinal expansion with the vortical motion being only a small correction on top of it. The vorticity field is calculated and illustrated in detail. The formation of two oppositely-rotating vortex rings moving in opposite directions along the $z$ axis is demonstrated. Other characteristics of the vortical motion such as the Lamb vector field and the kinematic vorticity number are considered. The magnitude of the latter one is found to be smaller than that for the Poiseuille flow and close to the pure shear deformation corresponding to just a flattening of fluid cells. The field of hydrodynamic helicity, which is responsible for the axial vortex effect, is calculated. The separation of positive and negative helicities localized upper and lower semi-planes with respect to the reaction plane is shown. It is proved that the areas with various helicity signs can be probed by the selection of $\Lambda$ hyperons with positive and negative projections of their momenta orthogonal to the reaction plane.