Global $\Lambda$ hyperon polarization in low-energy heavy ion collisions -- a scenario without vorticity

TL;DR

This paper proposes a novel mechanism linking transverse $\Lambda$ polarization in unpolarized collisions to global polarization in heavy-ion collisions, suggesting that flow-driven alignment can account for observed polarization without vorticity.

Contribution

It introduces a new explanation connecting transverse and global $\Lambda$ polarization phenomena, supported by Monte Carlo simulations at low energies.

Findings

The proposed mechanism can produce about 23% of the observed global polarization.

Simulations at $\sqrt{s_{NN}}=3$ GeV support the viability of the mechanism.

This work establishes a quantitative link between two polarization phenomena.

Abstract

Since its discovery, global polarization of the $\Lambda$ hyperon in heavy-ion collisions has been firmly established and is widely attributed to the large vorticity generated in the rotating quark-gluon plasma. In contrast, nearly fifty years after the first observation of unexpectedly large transverse $\Lambda$ polarization in unpolarized hadron collisions, its underlying mechanism remains an open and long-standing puzzle, despite being observed across a broad range of collision systems. Although these two phenomena exhibit notable similarities, they are generally regarded as arising from distinct physical origins. In this work, we propose a direct connection between $\Lambda$ global polarization in heavy-ion collisions and the long-standing transverse polarization observed in unpolarized collision systems. We demonstrate that the alignment between the $\Lambda$ production plane and the reaction plane, driven by directed flow, can transfer transverse polarization into the measured global polarization signal. Realistic Monte Carlo simulations of Au+Au collisions at $\sqrt{s_{\rm NN}} = 3$ GeV indicate that this mechanism can generate a sizable global polarization, accounting for approximately $23\%\pm6\%$ of the magnitude reported by the STAR Collaboration. Our results establish, for the first time, a quantitative link between these two well-known phenomena and have important implications for the interpretation of $\Lambda$ global polarization measurements in low-energy heavy-ion collisions.