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Abstract
Vortical structure of hot and dense matter created in heavy ion collisions can be probed through global polarization of emitted particles. Hyperon‘s weak decays provides opportunity to measure this phenomenon. Global polarization of Λ hyperons was measured by the STAR experiment at RHIC for Au+Au collisions with sqrt(s_NN ) = 3-200 GeV and at the LHC for Pb+Pb collisions with sqrt(s_NN ) = 2.76 and 5.02 TeV. Global polarization of multistrange hyperons, such as Ξ and Ω, can provide new information for hydrodynamic description of the system and its vorticity structure. In this letter, global polarization of Ξ^−, Ξ ̅^+, Ω^− and Ω ̅^+ hyperons was measured for Au+Au collisions at sqrt(s_NN ) = 7.7, 9.2, 11.5, 14.6, 17.3, 19.6 and 27 GeV, recorded with STAR at RHIC. We observed significant global polarization of Ξ^− and Ξ ̅^+ hyperons, which increases with decreasing centrality and decreases with increasing collision energy. The global polarization of Ω^−+Ω ̅^+ hyperons indicating collision energy dependence, with magnitudes exceeding those of Ξ and Λ. No significant dependence on rapidity or transverse momentum was observed.

Proposed figures

Figure 1

Fig.1: Global polarization of Ξ^-+Ξ ̅^+ hyperons as a function of centrality in Au+Au collisions at√(s_NN )= 7.7, 9.2, 11.5, 14.6, 17.3, 19.6, 27 GeV measured directly (blue marks) and via daughter global polarization (red marks). Open boxes and vertical lines show systematic and statistical uncertainties, respectively.

Figure2

Fig.2: Global polarization of Ξ^-+Ξ ̅^+ hyperons as a function of collision energies in Au+Au collisions at√(s_NN )= 7.7, 9.2, 11.5, 14.6, 17.3, 19.6, 27 GeV.

Figure 3

Fig.3 : Global polarization of Ω^-+Ω ̅^+ hyperons as a function of collision energies in Au+Au collisions at√(s_NN )= 7.7, 9.2, 11.5, 14.6, 17.3, 19.6, 27 GeV.

Supplemental Figure 1

Supplemental-Fig.1 : Global polarization of Ξ^-+Ξ ̅^+ hyperons as a function of pt in Au+Au collisions at√(s_NN )= 7.7, 9.2, 11.5, 14.6, 17.3, 19.6, 27 GeV.

Supplemental Figure 2

Supplemental-Fig.2: Global polarization of Ξ^-+Ξ ̅^+ hyperons as a function of y in Au+Au collisions at√(s_NN )= 7.7, 9.2, 11.5, 14.6, 17.3, 19.6, 27 GeV.

Summary

We present the results of global polarization measurements forXi and Omega hyperons in Au+Au collisions at √sNN = 7.7, 9.2, 11.5, 14.6, 17.3, 19.6, 27 GeV with BES-II data.

(1) Global polarization of Ξ^-+Ξ ̅^+ and Ω^-+Ω ̅^+ hyperons increases with collision energy decreases

(2) Global polarization of Ξ^-+Ξ ̅^+ hyperons increases towards peripheral collisions

(3) No significant pt and y dependence

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Paper proposal global polarization of Xi hyperon in AuAu collisions at BES-II