Global $\Lambda$-hyperon polarization in Au+Au collisions at $\sqrt{s_\mathrm{NN}}=3$ GeV

TL;DR

This paper reports the first measurement of substantial $\Lambda$ hyperon polarization at 3 GeV in Au+Au collisions, challenging existing models and suggesting new insights into the fluid dynamics and hadronic mechanisms in low-energy nuclear collisions.

Contribution

First measurement of hyperon polarization at 3 GeV, revealing significant polarization that prompts reevaluation of fluid viscosity and polarization mechanisms in low-energy collisions.

Findings

Measured polarization of 4.91% at 3 GeV

Indicates possible need to revise fluid viscosity models

Suggests hadronic mechanisms significantly contribute to polarization

Abstract

Global hyperon polarization, $\overline{P}_\mathrm{H}$, in Au+Au collisions over a large range of collision energy, $\sqrt{s_\mathrm{NN}}$, was recently measured and successfully reproduced by hydrodynamic and transport models with intense fluid vorticity of the quark-gluon plasma. While na\"{i}ve extrapolation of data trends suggests a large $\overline{P}_\mathrm{H}$ as the collision energy is reduced, the behavior of $\overline{P}_\mathrm{H}$ at small $\sqrt{s_\mathrm{NN}}<7.7$ GeV is unknown. Operating the STAR experiment in fixed-target mode, we measured the polarization of $\Lambda$ hyperons along the direction of global angular momentum in Au+Au collisions at $\sqrt{s_\mathrm{NN}}=3$ GeV. The observation of substantial polarization of $4.91\pm0.81(\rm stat.)\pm0.15(\rm syst.)$% in these collisions may require a reexamination of the viscosity of any fluid created in the collision, of the thermalization timescale of rotational modes, and of hadronic mechanisms to produce global polarization.