Hyperon Polarization from the Vortical Fluid in Low Energy Nuclear Collisions

TL;DR

This study investigates how hyperon polarization varies with beam energy in low energy nuclear collisions, identifying a peak around 7.7 GeV, indicating the most vortical fluid formation.

Contribution

It provides the first systematic analysis of hyperon polarization dependence on beam energy in the 1-10 GeV range, predicting a non-monotonic trend with a maximum at 7.7 GeV.

Findings

Hyperon polarization increases then decreases with decreasing beam energy.

Maximum polarization occurs around 7.7 GeV.

Predicted non-monotonic energy dependence of hyperon polarization.

Abstract

In 2017, STAR Collaboration reported the measurements of hyperon global polarization in heavy ion collisions, suggesting the subatomic fireball fluid created in these collisions as the most vortical fluid. There remains the interesting question: at which beam energy the truly most vortical fluid will be located. In this work we perform a systematic study on the beam energy dependence of hyperon global polarization phenomenon, especially in the interesting $\hat{O}(1\sim 10)\ \rm GeV$ region. We find a non-monotonic trend, with the global polarization to first increase and then decrease when beam energy is lowered from $27~\rm GeV$ down to $3~\rm GeV$. The maximum polarization signal has been identified around $\sqrt{s_{NN}} = 7.7~\rm GeV$, where the heavy ion collisions presumably create the most vortical fluid. Detailed experimental measurements in the $\hat{O}(1\sim 10)\ \rm GeV$ beam energy region are expected to test the prediction very soon.