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Paper proposal: Measurements of global polarization of Lambda hyperons in Au+Au collisions from the RHIC Beam Energy Scan-II

Updated on Fri, 2024-10-18 05:22. Originally created by futong on 2024-09-12 08:39.

Title: Measurements of global polarization of Λ/Λ ̅ hyperons in Au+Au collisions from the RHIC Beam Energy Scan-II

PAs: Egor Alpatov, Zhenyu Chen, Tong Fu, Xingrui Gou, Qiang Hu, Grigory Nigmatkulov, Hao Qiu, Subhash Singha, Qinghua Xu

Target Journal: PRC 

Previous study: 
https://drupal.star.bnl.gov/STAR/blog/futong/Global-polarization-Lambda-AuAu-collisions-77GeV
https://drupal.star.bnl.gov/STAR/blog/xgou/Global-polarization-AuAu-collisions-92-115-145GeV
https://drupal.star.bnl.gov/STAR/blog/futong/Global-polarization-Lambda-AuAu-collisions-173-GeV-BES-II
https://drupal.star.bnl.gov/STAR/meetings/STAR-Collaboration-Meeting-March-2024/FCV-parallel-session-II/Global-polarization-Lambda-hy
 

Abstract:
The observation of Λ and Λ ̅ global polarization along the system’s angular momentum has revealed the existence of large vorticities in the medium created by heavy-ion collisions. A splitting between Λ and Λ ̅ global polarization could originate from the strong late-stage magnetic field in the collision. In this paper, we present the Λ and Λ ̅ global polarization in Au+Au collisions at #sqrt{S_NN} = 7.7, 9.2, 11.5, 14.6, and 17.3 GeV from the RHIC Beam Energy Scan Phase II, using the updated STAR detector. The results show global polarization increasing with decreasing #sqrt{S_NN}) within 20-50% centrality over the whole energy range. No significant splitting is observed across a broad range of collision energies. The dependencies of global polarization on centrality, transverse momentum, and rapidity were measured. An indication of rapidity dependence in global polarization is observed at low collision energy. These results provide new insights into the polarization mechanism and vorticity fields in heavy-ion collisions, as well as additional constraints on the properties and dynamics of the matter created in the collisions.

Proposed figures

Figure 1:


 
Fig.1 : Global polarization (P_H) measurements are presented as a function of collision centrality at √sNN = 7.7, 9.2, 11.5, 14.6, and 17.3 GeV. The statistical uncertainties are displayed as lines, while the systematic uncertainties are shown as open boxes.



Figure 2:

Fig.2 : The midcentral (20-50%) P_H measurements reported in this work are shown in panel (a) alongside previous measurements and are consistent with the earlier results at the energies studied here. The difference between integrated P_Λ ̅  and P_Λ is displayed in panel (b) at √sNN = 7.7, 9.2, 11.5, 14.6, and 17.3 GeV alongside previous measurements. No significant difference in global polarization between Λ and Λ ̅  was observed at these energy levels.

Figure 3:




Fig.3 : Global polarization of Λ and Λ ̅ as a function of  p_T in Au+Au collisions from the RHIC BES-II，No dependence of P_H on p_T was observed at √sNN = 7.7, 9.2, 11.5, 14.6, or 17.3 GeV, consistent with previous observations.

Figure 4:


Fig.4 : Global polarization of Λ and Λ ̅ as a function of y_Λ/y_beam in Au+Au collisions from the RHIC BES-II. An indication of rapidity dependence in global polarization was observed at lower collision energies

Figure 5:

Fig.5 : Global polarization of Λ as a function of |y_Lambda| in Au+Au collisions from the RHIC BES-II. 



Conclusions:

1. We measured global polarization using BES-II data, which has five times less uncertainty than BES-I and includes two new energy points. No significant difference between Λ and Λ̅ polarization was observed in 20-50% centrality.

2. Global polarization dependencies on transverse momentum and rapidity were measured, with no significant transverse momentum dependence observed but an indication of rapidity dependence detected at low collision energy.

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