STAR Flow, Chirality and Vorticity PWG

[Takafumi Niida <niida AT bnl.gov>]

Date: 2/17/2023

Participants: Yicheng Feng, Fuqiang Wang, Hanna Zbroszczyk, Maria Zurek, Nihar Sahoo, Prithwish Tribedy, Subhash Singha, Toshihiro Nonaka, Yi Yang, Yue-Hang Leung, Rongrong Ma, Takafumi Niida

Title: Estimate of Background Baseline and Upper Limit on the Chiral Magnetic Effect in Isobar Collisions at sNN = 200 GeV at the BNL Relativistic Heavy-Ion Collider
PAs: Yicheng Feng, Fuqiang Wang, CME focus group 

Target journal: PRL, RPC
Proposal page: https://drupal.star.bnl.gov/STAR/blog/fengyich/Background-Baseline-CME-Isobar-Collisions-Webpage

Presentation: https://drupal.star.bnl.gov/STAR/system/files/IsobarNfBkg_PwgcPreview_v5.pdf

The PWGC panel previewed a paper proposal from FCV PWG. The panel found that the analysis is mature and the paper should move forward. The target journals are appropriate. The following points were discussed.

Slide 9

Q. It says “pion pair” and pT up to 2 GeV/c is used. How was the PID done?

A. We used TPC and TOF.

Q. For subevent, how did you make pairs? One from one subevent, and the other from another subevent? |\Delta\eta|>0.05 needs to be mentioned?

A. Two particles are taken from the same subevent requiring the eta gap, as used in 3PC measurements. So the label is true.

Slide 12

Q. How does the “true” flow look compared to v2{EPD}? Less non-flow in v2{EPD} right?

A. Haven’t compared to it. It would be closer but there still exist some non-flow as well as decorrelation effect. Contribution from flow fluctuation may be also different. This is one way to measure “true" flow. PAs will make such a comparison.

C. Please consider to rename “true” flow.

Q. What do you mean by “true” flow? Free from non-flow but still contains flow fluctuation?

A. Yes.

Slide 14

Q. Systematic uncertainties look very large in most peripheral bin. Why? Better to drop or need some explanation?

A. It comes from two close number difference and ratio. But it could be improved. PAs will check on this.

Q. Any correlation between the uncertainties?

C. It was already taken care of.

Slide 15

Q. The assumption of 5% for possible CME signal is based on what?

A. It’s a guess. CME signal if any would be a few % level. The 1sigma uncertainty is assigned.

Slide 16

Q. HIJING was used for this estimate. Did you try other model?  Is it worst to trust?

A. For estimate of non-flow, we wanted to use the model which doesn’t have flow, that was HIJING. At most peripheral bin, the data approaches HIJING calculations, which gives us confidence for using this model for non-flow estimate. We take the difference between two setups for conservative estimate.

Q. If you take a ratio of quenching-on to quenching-off, should it increases towards central?

A. Not sure what we should expect because of complication, e.g. the correlation may decrease with quenching-on by dilution effect of the correlation due to larger multiplicity.

C. It would be better to spend one paragraph to explain HIJING how good/bad it is to describe the data.

C.There is Raa in isobars from Tong Liu. It would be good to make the comparison of Raa with HIJING just to see how compatible with data.

Slide 22

Q. Some of the data points have no baseline. Why?

A. Those used EP-method. Our estimate is applicable to 2PC method, though it should be similar for EP-method.

Slide 23

Q. What is the upper limit for? Isobar difference?

A. It is fCME for Zr.

Q. Why 98% confidence level instead of 90% or 95%? 

A. It is equivalent to 2\sigma on one side of Gaussian.

Q. 4 different upper limits from different groups are quoted. Should we just quote smallest one, 9.6%?

A. No, they are based on different cuts and different techniques. 

C. Instead of 10%, probably better to say 9.6-11%.

Q. Do we need to put institution names in the table?

A. This is only for internal purpose.