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

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 naïve 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 $\hat{I}›$ 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 $4.91\pm 0.81(stat.)\pm 0.15(syst.)$% 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.

9 pages, 6 figures