$\Lambda$ spin polarization in event-by-event relativistic heavy-ion collisions

TL;DR

This paper uses event-by-event relativistic hydrodynamics to study $ ext{Lambda}$ hyperon polarization, exploring effects of initial conditions, shear viscosity, and system size, and proposing new correlations to test polarization mechanisms.

Contribution

It provides the first detailed event-by-event predictions of $ ext{Lambda}$ polarization Fourier coefficients and introduces new correlation observables to probe polarization origins.

Findings

Predicted Fourier coefficients of $ ext{Lambda}$ polarization for different flow planes.

Identified the impact of initial hot spot size and shear viscosity on polarization.

Proposed new correlations between polarization and flow coefficients.

Abstract

We present a systematic study of $\Lambda$ hyperon's polarization observables using event-by-event (3+1)D relativistic hydrodynamics. The effects of initial hot spot size and QGP's specific shear viscosity on the polarization observables are quantified. We examine the effects of the two formulations of the thermal shear tensor on the polarization observables using the same hydrodynamic background. With event-by-event simulations, we make predictions for the Fourier coefficients of $\Lambda$'s longitudinal polarization $P^z$ with respect to the event planes of different orders of anisotropic flow. We propose new correlations among the Fourier coefficients of $P^z$ and charged hadron anisotropic flow coefficients to further test the mapping from fluid velocity gradients to hyperon's polarization. Finally, we present a system size scan with Au+Au, Ru+Ru, and O+O collisions at $\sqrt{s_\mathrm{NN}} = 200$ GeV to study the system size dependence of polarization observables at the Relativistic Heavy-ion Collider.