Effect of the spin-orbit interaction on flows in heavy-ion collisions at intermediate energies

TL;DR

This study investigates how spin-orbit interactions influence flow patterns in heavy-ion collisions using an advanced UrQMD model, revealing negligible net effects but notable spin-dependent flow splitting at specific energies and impact parameters.

Contribution

It introduces the inclusion of spin-orbit coupling effects into the UrQMD model and analyzes their impact on nucleon flow in heavy-ion collisions, highlighting the importance of spin degrees of freedom.

Findings

Negligible net contribution of spin-orbit coupling to flows.

Observable flow splitting between spin-up and spin-down nucleons.

Flow splitting comparable to symmetry energy effects, affecting neutron flow analysis.

Abstract

The effect of the spin-orbit coupling in heavy ion collisions is investigated based on an updated version of the ultra-relativistic quantum molecular dynamics (UrQMD) model, in which the Skyrme potential energy density functional is employed. And in special, the spin-orbit coupling effects on the directed and elliptic flows of free nucleons emitted from $^{197}$Au+$^{197}$Au collisions as functions of both the beam energy and the impact parameter are studied. Our results show that the net contribution of the spin-orbit term to flows of nucleons is negligible, whereas a directed flow splitting between spin-up and spin-down nucleons is visible especially at large impact parameters and a peak of the splitting is found at the beam energy around 150 MeV$/$nucleon. We also found that the directed flow splitting between spin-up and spin-down neutrons is comparable with the neutron directed flow difference calculated by a soft and a stiff symmetry energy, indicating that the directed flow of neutrons cannot be used to pin down the stiffness of symmetry energy any more without considering the spin degree of freedom in models in case of spin polarization.