STAR Flow, Chirality and Vorticity PWG

[Sooraj Radhakrishnan <skradhakrishnan AT lbl.gov>]

Date: 01/31/2025

Participants: Jiangyong Jia, Jinhui Chen, Shengli Huang, Zhengxi Yan, Xiaoxuan Chu, Hanna Zbroszczyk, Nu Xu, Prithwish Tribedy, Richard Seto, Subhash Singha, Isaac Mooney, Nihar Sahoo, ShinIchi Esumi, Sooraj Radhakrishnan

Title: Measurements of Multiplicity Dependent v2 and v3 in O+O and d+Au Collisions

PAs: Shengli Huang, Jinhui Chen, Roy Lacey, Jiangyong Jia, Prithwish Tribedy, Zaining Wang, Zhengxi Yan, Chunjian Zhang

Target journal: PRL or Nature Physics

Proposal page: https://drupal.star.bnl.gov/STAR/blog/slhuang/PaperFlowindAuandOORun21

Presentation: https://drupal.star.bnl.gov/STAR/blog/slhuang/PWGC-Slide-OO-and-dAu-flow-paper

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 make the physics message more focused. The following points were discussed.

Q: In Figs.2 and 4, what is the difference between the two models? What does it mean one model describe data or not?

A: Different models describe Fig,2 and Fig.4. So there is no clear preference which one is better, but this tells models need to improve to describe the eccentricity and fluctuations 

Q: How do the results presented answer questions raised on S.4?

A: If the v2 are strongly dependent on geometry, then it is final state effect and not initial state effects. From the data, Fig.1 and Fig.2 we see strong geometry dependence with the d+Au v2 being much larger at the same multiplicity than O+O. 
A. Also from Fig.2 it can be seen that the eccentricity scaled values from both systems are similar, as in the large systems where hydrodynamics describe the vn

Q: Wasnt this conclusion also reached in the PHENIX paper?

A: In the PHENIX v2 measurement comparing p+Au and d+Au, the difference is small. For v3 between the different small systems, the difference could be explained by sub-nucleon fluctuations. The current measurements give clear evidence of sensitivity to geometry. Also, effect of decorrelation need to be taken into account in PHENIX measurement, which makes interpretation complicated 
A: Also, with iTPC, we can now go to higher multiplicities for d+Au, where the difference is larger 
C: This point should be made more clear

C: Another point is, with the small difference seen in previous measurements, non-flow and non-flow subtraction can complicate interpretations. In this case the difference is clear even before any non-flow subtraction 
A: We can clarify these 

Q: Im Fig.1, is the acceptance range < 1.5? Is the x-axis corrected for efficiency?
A: Yes, the x-axis (Ntrack) is corrected for efficiency. Yes, the acceptance range is 1.5 
Q: Ho was the correction done?
A: Using embedding for evaluation of the efficiency

Q: Why the v2 values from the two systems dont converge at higher multiplicities?
A: Because even in most central collisions, d+Au has a geometry giving large v2  

Q:  Do we have p+Au results? Wouldnt it be better to use p+Au instead of d+Au

A:  No, d+Au is better because of inherent geometry in the collisions. The exisiting p+Au data also cannot go to this high multiplicties 

Q: What is the model used for d+Au in Fig,.4? 

A: It is from Galuber
Q: Please have the lagends for O+O moved to the right plot
A: Yes, will do 

Q: Are these findings confirming that PHENIX conclusions wer right?
A: No. PHENIX measurements doesnt have the sensitivity to make these conclusions 

Q: S.23, can the conclusions be simplified? What are the conclusions?
A: Yes, will do. The main conclusion is that v2 are drive by geometry and final state interactions 

Q: S.23, the last sentence is not helpful in the summary
A: Yes, will remove 

Q: v3 is not mentioned after Fig.2. But it is prominently presented in the abstract

A: v3 shows the importance of sub-nucleon fluctuations 
C: It is important to make this point clear 

Q: Can we have Au+Au measurements to compare with these systems? Wouldnt Au+Au be more natural to use a baseline for system with flow?

A: We have only Run19 Au+Au with similar acceptance. But the statistics is small there. In similar multiplicity Au+Au also, the interpretation is not straightforward 

Q: Can we make any conclusion on QGP formation or why we dont see energy loss in small systems?
A: We can say v2 is from final state interactions, other questions are not what we focus on this measurement

Q: Is there value in showing the different model calculations in Fig.4? Because we dont have sensitivity to the presence of alpha clusters etc?
A: Yes, we will simplify these figures 

Q: On S.6, the d+Au eccentricity doesnt increase with multiplicity at high multiplicity, but the v2 increases. Why is this?
A: This is from the medium response increasing with increasing multiplicity

Q: The physics message from Figs.3 and 4 are not clear
A: These are looking at eccentricity fluctuations. We will sharpen the physics message 

Q: On Fig.1a, why both d+Au and O+O show a kink at similar multiplicity, around 30 tracks?
A: This is from non-flow dominance 
Q: Why the eccentricity also shows similar increase to lower multiplicities? 
A: This is from smaller v2 from medium response due to lower multiplicities  

--

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