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  • Open Access

Global Λ hyperon polarization in low-energy heavy ion collisions: A scenario without vorticity

Feng Liu1,* and Zhoudunming Tu1,2,†

  • *Contact author: feng.liu.2@stonybrook.edu
  • †Contact author: zhoudunming@bnl.gov
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Phys. Rev. D 113, 114051 – Published 30 June, 2026

DOI: https://doi.org/10.1103/c4nr-7qj4

Abstract

Since its discovery, global polarization of the Λ 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 Λ 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 Λ global polarization in heavy-ion collisions and the long-standing transverse polarization observed in unpolarized collision systems. We demonstrate that the alignment between the Λ 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 sNN=3  GeV indicate that this mechanism can generate a sizable global polarization, accounting for approximately 23%±6% 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 Λ global polarization measurements in low-energy heavy-ion collisions.

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Phys. Rev. D 113, 114051– Published 30 June, 2026
  • Received 9 April 2026
  • Accepted 3 June 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.

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