Title: Global polarization of Xi hyperons at √s_NN = 14.6, 19.6 and 27 GeV in Au+Au collisions

PAs: Egor Alpatov (NRNU MEPhI), Grigory Nigmatkulov (UIC)
Contact information: egroker1@gmail.com, gnigmat@uic.edu

Target journal: PRL

Talks in PWG:

• March 13 2023: Paper Proposal
• Dec 1 2021: 14.6 GeV
• Oct 20 2021: 19.6 GeV
• 30 Sep 2020: 27 GeV
• 17 Sep 2020: 27 GeV
• Aug 5 2020: 27 GeV
• July 22 2020: 27 GeV

Collaboration Meetings:

• 20 March 2023: STAR Analysis Meeting
• Jun 28 2023: BES-II Meeting, 19.6 & 27 GeV
• Feb 16 2022: 19.6 & 27 GeV
• Sep 15 2021: 19.6 GeV
• Sep 17 2020: 27 GeV

Tracking Meeting:
       Dec 21 2020: KFParticle and topology comparison in 27 GeV
Preliminary figures

• 27 GeV Preliminary figures
• 19.6 GeV Preliminary figures

 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 √(s_NN )= 3-200 GeV and at the LHC for Pb+Pb collisions with √(s_NN )= 2.76 and 5.02 TeV.
Global polarization of multistrange hyperons, such as Ξ, can provide new information for hydrodynamic description of the system and its vorticity structure.
In this letter, global polarization of "Ξ" ^- and "Ξ"  ̅^+ hyperons was measured for Au+Au collisions at √(s_NN )= 14.6, 19.6 and 27 GeV recorded with STAR at RHIC. No significant rapidity or transverse momentum dependence was observed. "Ξ" global polarization increases towards peripheral collisions and decreases with collision energy increases.

Motivation:
STAR published Λ polarization for 7.7-200 GeV collider mode and 3 GeV in FXT .
While theory was able to describe integral measurements of Λ global polarization, there is still lackof experimental input for differentialmeasurements and other hyperon species.
We present measurements for globalpolarization of Ξ hyperons at √(s_NN )=14.6, 19.6 & 27 GeV and Λ global polarization at √(s_NN )=14.6 GeV


Figure 1: 

Centrality dependence of Ξ hyperons for Au+Au collisions at √(s_NN )=14.6, 19.6 and 27 GeV measured directly (blue triangles) and via daughter global polarization (red boxes), in comparison with inclusive Λ global polarization (green circles).

Figure 2:

p_T dependence of Ξ hyperons for Au+Au collisions at √(s_NN )=14.6, 19.6 and 27 GeV measured directly (blue triangles) and via daughter global polarization (red boxes), in comparison with inclusive Λ global polarization (green circles).

Figure 3:
y dependence of Ξ hyperons for Au+Au collisions at √(s_NN )=14.6, 19.6 and 27 GeV measured directly (blue triangles) and via daughter global polarization (red boxes), in comparison with inclusive Λ global polarization (green circles).

Figure 4:

Energy dependence of Ξ global polarization in √(s_NN )=14.6, 19.6 and 27 GeV Au+Au collisions, measured directly and via daughter global polarization.
Bands represent AMPT predictions [1]. Black markers states inclusive Λ global polarization from BES-I and BES-II [2,3,4], and previous Ξ global polarization measurements at √(s_NN )=200 GeV [5].
Markers for Ξ and BES-II Λ measurements at √(s_NN )=14.6, 19.6, 27 and 200 GeV are moved for better visibility.

[1] PLB 827 (2022) 136971

[2] Nature 548, 7665, p. 62–65 (2017)

[3] PRC 98, 014910 (2018)

[4] PRC 108, 014910 (2023)

[5] PRL 126, 162301 (2021)

Conclusions:

We presented measurements of global polarization of Ξ^-/Ξ ̅^+ at √(s_NN )=14.6, 19.6 and 27 GeV and global polarization of Λ/Λ ̅ at √(s_NN )=14.6 GeV

• Global polarization of Ξ hyperons increases with collision energy decreses
• Global polarization of Ξ hyperons increases towards peripheral collisions
• No significant p_T or y dependence observed for global polarization of Ξ^-+Ξ ̅^+