STAR Flow, Chirality and Vorticity PWG

[Sooraj Radhakrishnan <skradhakrishnan AT lbl.gov>]

Date: 02/28/2025

Participants: 
Egor Alpatov, Grigory Nigmatkulov, Qiang Hu, Qinghua Xu, Tong Fu, Xingrui Gou, Xianglei Zhu, Jae Nam, Xiaoxuan Chu, Hanna Zbroszczyk, Nu Xu, Xin Dong, Prithwish Tribedy, Richard Seto, Subhash Singha, Isaac Mooney, Nihar Sahoo, Qian Yang, (Tommy) Chun Yuen Tsang,  Guannan Xie, Yue Hang Leung, Barbara Trzeciak, ShinIchi Esumi, Sooraj Radhakrishnan

Title: Global polarization of 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: PRL (short), PRC (long)
Proposal page: https://drupal.star.bnl.gov/STAR/blog/futong/Paper-proposal-Global-polarization-hyperons-AuAu-collisions-RHIC-Beam-Energy-Scan-II
Presentation: https://drupal.star.bnl.gov/STAR/system/files/PWGC_preview_2.pdf

The PWGC panel previewed a paper proposal from FCV PWG. The panel found that the analysis is mature and results are important and interesting, and the paper should move forward. The journal choices were also found to be appropriate, although was suggested to sharpen the physics message for PRL. The following points were discussed 

Q: In the conclusions for PRL, what would be the physics conclusions? In addition to the observations quoted
A: This is the first measurement of Xi and Omega global polarization energy dependence. There is no significant difference between the polarization of baryons with different strangeness content. Within the current uncertainties the polarizations are consistent 

Q: Did you check the stability of the results in different Vz regions? You use a large Vz selection range
A: Yes, we can do that. With the large Vz selection we can extend the study of the rapidity dependence 

Q: We did publish the Lambda - Lambdabar polarization difference at 19.6 and 27 GeV. How does the precision from this measurement compare to the previous measurement?
A: The precision is comparable. This extends the measurements to lower energies
C: 19.6 and 27 GeV probably has better precision
Q: Wasnt the cascade measurements done at 200 GeV? Didnt we see the results are comparable to that of Lambda there?
A:  Yes, but the precision is much less at 200 GeV

Q: Can we conclude if there is difference or not between Lambda and Cascade polarizations from our measurements? The difference predicted in AMPT is of the order of our uncertainties. If not how can we make a physics statement on the comparison?
A:  We can do some statistical test and quote a p value for the difference. Making a physics conclusion might be more difficult 

Q: We dont see difference between Lambda and Lambdabar polarization. How do we understand this?
A: The polarization is sensitive to the late stage magnetic field. In the previous PRC also we quoted an upper limit for the late stage magnetic field. We could quote a number from a similar analysis, but a more detailed calculation taking into account different contributions can be difficult

Q: Can we include the rapidity dependence to the PRL paper?
A: Yes, we will consider this

Q: Do you have other models that look into the difference in polarization between different species?
A: We shall ask the theorists for the calculations 

Q: For the rapidity dependence can we see at which energy we see rapidity dependence?
A: At higher energies we dont see rapidity dependence.
Q: Can we quantify the significance of this non uniformity?
A: Yes, we will do
C: You could swap current Fig.3 in PRL with the rapidity dependence. Do you have a physics message for Fig.3?
A: Yes, the error bars are large for Omega. We will consider making this change
C: Could include the Omega results into a second panel of Figure 2

Q: How is Omega polarization measured?
A: We measure through the polarization of Lambdas. For cascade we measured directly using daughter Lambdas and from the polarization of Lambdas

Q: In Fig.2 which method is used?
A: Both methods are shown here with different markers 

Q: In the measured Lambda polarization there is contribution from feeddown from Cascade and Omega?
A: We apply selection cuts to remove Lambda decayed from Cascade and Omega in the analysis of Lambda polarization. The feeddown contribution is small, less than 10%. There is only model dependent way to evaluate this

Q: In the AMPT model, where is the Lambda without feeddown
A: We had this in previous version of the figure, will include

Q: How does UrQMD calculations without hydro look like in Figure 1? How does hydro change the values? For low energies does hydro work?
A:  We can include this calculation

Q: Th abstract discusses splitting bwteen Lambda and Lambda bar, can be removed. We dont also address the difference between the particles
A:  Yes, will consider changing

Q: The notation for the labels on S.10 is confusing, please consider a better way
A: Yes, will do

Q: The indirect measurement is supposed to have better precision. It is also seen at 200 GeV. Why are the error bars comparable in Fig.10
A: The error bars are better for the indirect measurement in Fig.10 also
Q: In the differential measurements in S.21, the difference seems starker

Q: In Fig.2 (PRL), are the two methods supposed to give same values? There seem to be difference at 200 GeV isobar measurements and at BES-II energies
A: They are supposed to give same values. The difference seen so far are not significant 

Q: In the isobar polarization measurement, it is claimed that cascade polarization is higher than that of lambda, even though the significance is less than 3 sigma. Here we claim there is no difference. Can we make the conclusions consistent between both papers?
A:  Yes, will check

Q: The feeddown fraction for lambda strongly depends on energy. So if we say the contribution is small, please do double check the numbers
A: Yes, will do

--

Sooraj Radhakrishnan

Research Scientist,

Department of Physics

Kent State University

Kent, OH 44242

Physicist Postdoctoral Affiliate

Nuclear Science Division
Lawrence Berkeley National Lab
MS70R0319, One Cyclotron Road

Berkeley, CA 94720

Ph: 510-495-2473

Email: skradhakrishnan AT lbl.gov