Hyperon global polarization in heavy-ion collisions at NICA energies. Feed-down effects and the role of $Σ^0$ hyperons

Abstract

Global polarization of hyperons induced by the local vorticity of the medium created in heavy-ion collisions at energies 2.3\,GeV$\le\sqrt{s_{NN}}\le$11.5 GeV is calculated in the parton-hadron-string dynamic (PHSD) model. The separation of spectator nucleons and the fluidization of the generated particle distributions are performed. The polarization of all anti-hyperon species is found significantly larger than that of hyperons. The $\overlineΞ$ hyperons are found to be polarized as strong as $\overlineΛ$s but $Ξ$ hyperons have weaker polarization compared to $Λ$s. The $Ω$ and $\overlineΩ$ polarizations show the strongest dependence on the collision energy. Despite the strong polarization of the produced $Λ$s and $\overlineΛ$s induced by the vortical flows in the medium, the observed polarization signal is significantly depleted because of the feed down from weak and electromagnetic decays of heavier hyperons. Particularly strong suppression is found to be due to electromagnetic decays of $Σ^0$ hyperons, which multiplicities obtained in the transport are poorly constrained both from the microscopic input of the $Σ^0$ production reactions and from the experimental data. The final $Λ(\overlineΛ)$ polarization signal strongly depends on the $Σ^0$ multiplicity generated in the model. With all these effects we can reproduce the measured global $Λ$ polarization in collisions at $\sqrt{s_{NN}}=7.7$ and $11.5$ GeV and the global $\overlineΛ$ polarization at 11.5 GeV. For energies $< 3$ GeV, the calculated $Λ$ polarization is smaller than the observed one. The polarization of $Ξ(\overlineΞ)$ hyperons is calculated. The signal of $Ξ(\overlineΞ)$ polarization is argued to be insensitive to feed-down effects and be a more direct probe of the degree of the vorticity in the system.