STAR Flow, Chirality and Vorticity PWG

[Takafumi Niida <niida AT bnl.gov>]

Date: 10/21/2022

Participants: Xiaoyu Liu, Mike Lisa, ShinIchi Esumi, Daniel Cebra, Daniel 
Brandenburg, Hanna Zbroszczyk, Jiangyong Jia, Maria Zurek, Nihar Sahoo, 
Prithwish Tribedy, Qinghua Xu, Sooraj Radhakrishnan, Subhash Singha, Xiaoxuan 
Chu, Yi Yang, Takafumi Niida, Rongrong Ma

Title: Measurement of directed flow at forward and backward pseudorapidity in 
Au+Au collisions at GeV with the Event Plane Detector (EPD) at STAR
PAs: Xiaoyu Liu, Shinichi Esumi, Mike Lisa, Prithwish Tribedy
Target journal: PRC
Proposal page: 
https://drupal.star.bnl.gov/STAR/blog/lxy1122/v1-measurement-EPD-27-GeV
Presentation: 
https://drupal.star.bnl.gov/STAR/system/files/PWGC_preview_v1_27Gev.pdf

The PWGC panel previewed a paper proposal from FCV PWG. The panel found that 
analysis is mature and interesting, and agrees that the paper should move 
forward with the proposed journal. The following points are discussed.

Slide 7
Q. How good is TPC Psi1 resolution?
A. Something like ~0.1 but need to check.

Slide 9
Q. What does “0.29%” mean actually? relative uncertainties or absolute one? 
Seems too small.
A. The difference between orange and default is 0.0029, so it’s absolute 
value but “%” made confusion.

Figure 3
Q. What happens for nMip distribution for He3 fragment? Is there possibility 
to know its contribution? 
A. It may create strong peak at nMip~3 or 4. One may be able to see the 
effect in lower multiplicity/energy.

Figure 5
Q. Around eta = -4.5 (4.5), the data go up (down) while the UrQMD cannot 
describe the trend. Do you know the reason?
A. There is little information in such very forward/backward region. We would 
say that the models fairly do good job. The results will provide new insights 
on fragmented protons.
C. It would be nice to emphasize the point.

Q. What is the physics message here by comparing to UrQMD?
A. The difference between RP vs EP UrQMD tells us the effect of fluctuations 
relative to the impact parameter direction.
Also, momentum conservation effect seems to affect if it's closer to 
midrapidity (|eta|<|ybeam| region). But the agreement between data and 
UrQMD-EP may be accidental. We cannot say from the data what effects/how much 
they are.

Figure 7
Q. Do you know if the scaling holds for other centrality?
A. We have other centrality data for STAR but not for other experiment, so we 
don't know. But we can check using other BES-2 datasets in near future.

Q. That limiting fragmentation scaling also holds other centrality within 
STAR data?
A. Haven’t checked but probably not. Since dN/deta is different for different 
centrality, it wouldn’t hold for a fixed energy.

Q. There seems slight different trend compared to PHOBOS 19 GeV. Midrapidity 
as well as beam rapidity region. Why?
A. At midrapidity, pT range is different and energy is also slightly 
different. Around eta-ybeam=-1.2, indeed there is difference (maybe 1-2 sigma 
difference?). We are not sure which is right or wrong but also the location 
of minimum could be different for different energy where our detector cannot 
measure the whole trend.
Q. Around eta-ybeam = -0.2, there seem two slopes or jump.
A. We think it’s due to statistical fluctuations. 

Q. For EPD region, can we say we measure v1 down to pt=0?
A. Yes, we measure v1 for full pT with geant sim corrections.

Other comments/questions
C. Abstract says “present v1 over ten units of eta”, which is not true for 
BES-2 data. You have large gap in midrapidity. Please rephrase it better.
A. Will do. We could say 6 unit if we include midrapidity covered by TPC.

Q. Any plan to add midrapidity data from BES-2? BES-1 and BES-2 midrapidity 
v1 are consistent?
A. Not for this paper, but there is another paper which covers midrapidity 
and will be submitted soon. Yes, BES-1 and BES-2 v1 are consistent.

C. It might be useful to add flow chart as in backup slide and/or 
intermediate step for unfolding/correction of v1.