Home » Blogs » futong's blog

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

Updated on Wed, 2025-01-22 02:52. Originally created by futong on 2025-01-22 00:09.

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

Xi:
Egor Alpatov:
https://drupal.star.bnl.gov/STAR/system/files/Global_Polarization_22_07_2020.pdf
https://drupal.star.bnl.gov/STAR/system/files/Meeting_05_08_2020_0.pdf
https://drupal.star.bnl.gov/STAR/system/files/STAR_Meeting_17_09_Egor_0.pdf
https://drupal.star.bnl.gov/STAR/system/files/FCV_Alpatov_30Sep.pdf
https://drupal.star.bnl.gov/STAR/system/files/Hyperon%20global%20polarization%202021_0.pdf
https://drupal.star.bnl.gov/STAR/system/files/Hyperon%20global%20polarization%202021_2.pdf
https://drupal.star.bnl.gov/STAR/system/files/GlPol_14p6_FCV_dec.pdf
https://drupal.star.bnl.gov/STAR/system/files/Global%20%CE%9B%20and%20%CE%9E%20polarization.pdf
https://drupal.star.bnl.gov/STAR/system/files/Global%20%CE%9E%20polarization%202023.pdf
https://drupal.star.bnl.gov/STAR/system/files/Update%20on%20GlobPol%20fcv%2013%20March.pdf
https://drupal.star.bnl.gov/STAR/blog/alpatov/Proposal-and-Update-Global-Polarization-Xi
https://drupal.star.bnl.gov/STAR/blog/alpatov/kfparticle-and-helix
Paper proposal: Global polarization of Xi hyperons at sqrt(s_NN)=14.6, 19.6 and 27 GeV in Au+Au collisions:
https://drupal.star.bnl.gov/STAR/blog/alpatov/Paper-proposal-Global-polarization-Xi-hyperons-sqrtsNN146-196-and-27-GeV-AuAu-collisi-0
Xingrui Gou:
https://drupal.star.bnl.gov/STAR/system/files/Merasutemnt%20Xi%20global%20polarization%20in%20AuAu%207.7%2C%209.2%2C%2011.5%2C%2014.5%2017.3GeV.pdf
https://drupal.star.bnl.gov/STAR/system/files/20240620_Update%20Merasutemnt%20Xi%20global%20polarization%20in%20AuAu%207.7%2C%209.2%2C%2011.5%2C%2014.5%2017.3GeV_0.pdf
https://drupal.star.bnl.gov/STAR/system/files/20240705_Update%20measurement%20Xi%20global%20polarization%20in%20AuAu%207.7%2C%209.2%2C%2011.5%2C%2014.5%2017.3GeV.pdf
https://drupal.star.bnl.gov/STAR/blog/xgou/Update-Xi-global-polarization-centrality-pt-and-y-dependence-and-Omega-polarization-AuAu-B
https://drupal.star.bnl.gov/STAR/blog/xgou/Slides-Paper-Proposal-Xi-and-Omega-global-polarization-request-AuAu-collisions-BES-II
https://drupal.star.bnl.gov/STAR/system/files/20240717_Update%20Measurement%20Xi%20global%20polarization%20in%20AuAu%207.7%2C%209.2%2C%2011.5%2C%2014.5%2017.3GeV.pdf
https://drupal.star.bnl.gov/STAR/system/files/20240717_Measurement%20Omega%20global%20polarization%20in%20AuAu%207.7%2C%209.2%2C%2011.5%2C%2014.5%2017.3GeV_0.pdf

---------------------
Short paper

Title: Measurements of global polarization of Lambda, Xi and Omega hyperons in Au+Au collisions from the RHIC Beam Energy Scan-II

Target Journal: PRL 

Abstract:

The observation of global polarization along the system’s angular momentum has revealed the existence of large vorticities in the medium created by heavy-ion collisions. The weak decays of hyperons provide an opportunity to measure this phenomenon. A splitting between Λ and Λ̅ global polarization could originate from the strong late-stage magnetic field in the collision. The global polarization of multistrange hyperons, such as Ξ and Ω, can provide new insights into the hydrodynamic description of the system and its vorticity structure. In this paper, we present the global polarization of Λ, Ξ, and Ω in Au+Au collisions from the RHIC Beam Energy Scan Phase II, using the updated STAR detector. The results show that the global polarization of Λ and Ξ increases with decreasing sqrt{S_NN} within 20-50% centrality across the entire energy range. No significant splitting between Λ and Λ̅ global polarization is observed across a broad range of collision energies. The global polarization of Ω⁻ + Ω̅⁺ hyperons indicates a collision energy dependence, with magnitudes exceeding those of Ξ and Λ.

Proposed figures
 
Figure 1:

Fig.1 : 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 2:

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.

Conclusions:
 
1. We measured global polarization of Lambda and anti-Lambda 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. We present the results of global polarization measurements for Xi 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

------------------------

Long paper:

Title: Centrality, transverse momentum and rapidity dependence of Lambda and Xi global polarization in Au+Au collisions at RHIC BES-II

Target Journal: PRC 

Abstract:

The observation of global polarization along the system’s angular momentum has revealed the existence of large vorticities in the medium created by heavy-ion collisions. The weak decays of hyperons provide an opportunity to measure this phenomenon. The global polarization of multistrange hyperons can offer new insights into the hydrodynamic description of the system and its vorticity structure. In this paper, we present the dependencies of global polarization of Λ and Ξ on centrality, transverse momentum, and rapidity in Au+Au collisions from the RHIC Beam Energy Scan Phase II, using the updated STAR detector. We observe an indication of rapidity dependence in the global polarization of Λ at low collision energy. We also observe significant global polarization of Ξ hyperons, which increases with decreasing centrality. No significant dependence on rapidity or transverse momentum of Ξ was found. 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 : 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 3:

Fig.3 : 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 4:

Fig.4: 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.


Figure 5:

Fig.5 : 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. 

Figure 6:



Fig.6: 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.

Conclusions:
1. Global polarization of Lambda dependencies on centrality, transverse momentum and rapidity were measured, with no significant transverse momentum dependence observed but an indication of rapidity dependence detected at low collision energy.
2. Global polarization of Xi dependencies on centrality, transverse momentum and rapidity were measured, with no significant transverse momentum or rapidity dependence observed.

 

 

»