Probing early-time longitudinal dynamics with the $\Lambda$ hyperon's spin polarization in relativistic heavy-ion collisions

TL;DR

This paper investigates how the initial longitudinal flow velocity affects hyperon polarization in heavy-ion collisions, using hydrodynamic simulations constrained by experimental data to understand angular momentum transfer and fluid vorticity.

Contribution

It introduces a method to constrain initial longitudinal flow and angular momentum in hydrodynamic models using hyperon polarization and pion flow data.

Findings

Initial orbital angular momentum in the QGP is about 100-200 ℏ.

The initial longitudinal flow can be constrained by polarization and flow data.

Gradients of μ_B/T influence the polarization difference between Λ and anti-Λ.

Abstract

We systematically study the hyperon global polarization's sensitivity to the collision systems' initial longitudinal flow velocity in hydrodynamic simulations. By explicitly imposing local energy-momentum conservation when mapping the initial collision geometry to macroscopic hydrodynamic fields, we study the evolution of systems' orbital angular momentum (OAM) and fluid vorticity. We find that a simultaneous description of the $\Lambda$ hyperons' global polarization and the slope of pion's directed flow can strongly constrain the size of longitudinal flow at the beginning of hydrodynamic evolution. We extract the size of the initial longitudinal flow and the fraction of orbital angular momentum in the produced QGP fluid as a function of collision energy with the STAR measurements in the RHIC Beam Energy Scan program. We find that there is about 100-200 $\hbar$ OAM that remains in the mid-rapidity fluid at the beginning of hydrodynamic evolution. We further exam the effects of different hydrodynamic gradients on the spin polarization of $\Lambda$ and $\bar{\Lambda}$. The gradients of $\mu_B/T$ can change the ordering between $\Lambda$'s and $\bar{\Lambda}$'s polarization.