STAR Flow, Chirality and Vorticity PWG

[Takafumi Niida <niida AT bnl.gov>]

Date: 6/17/2022

Participants: Joseph Adams, Egor Alpatov, Michael Lisa, Grigory Nigmatkulov, Barbara Trzeciak, Daniel Brandenburg, Daniel Cebra, Jiangyong Jia, Maria Zurek, Matt Posik, Nihar Sahoo, Prithwish Tribedy, Qinghua Xu, Subhash Singha, Xiaofeng Luo, Yi Yang, Rongrong Ma, Takafumi Niida

Title: Global polarization of Lambda and anti-Lambda hyperons in Au+Au collisions at \sqrt{s_{NN}} =19.6 and 27 GeV

PAs: Joseph Adams, Egor Alpatov, Michael Lisa, Grigory Nigmatkulov

Target journal: PRC

Proposal page: https://drupal.star.bnl.gov/STAR/blog/adams92/Global-polarization-%CE%9B-and-%CC%84%CE%9B-hyperons-AuAu-collisions-%E2%88%9AsNN-196-and-27-GeV

Presentation: https://drupal.star.bnl.gov/STAR/system/files/LambdaPolarizationSplittingAt19And27GeV_Preview_v3.pdf

The PWGC panel previewed a paper proposal from FCV PWG. The panel agreed that the analysis is mature and should move forward. PRC as a target journal is appropriate and PLB is also mentioned for consideration. The following points were discussed.

Slide 7

Q. The v1 weight is prepared for each centrality bins?

A. Yes.

C. Since this analysis has some technical detail, consider to include such discussion and any EPD performance plot in the paper, e.g. EP resolution.

Slide 11

Q. TOF was used in the analysis? How was this effect checked?

A. Yes, TOF is used in KFP. 4 different sets (with/without TOF hits of daughters) were checked and no significant difference in the polarization (see slide 41).

Slide 14

Q. Why \overline is needed for PH? Polarization is measured as an ensemble.

A. Will consider to remove it.

Q. Efficiency correction is applied for pT-integrated or y-integrated results? No low-pT particles at forward rapidity as shown in slide 8. That may affect the rapidity dependence.

A. Since no significant pT/y dependence is observed, the correction is not applied. The effect would be negligible.

C. Still good to check it. For example, one can apply larger pT_min cut or narrower y-cut to see how the results change. Or estimate the uncertainty considering the observed/suggested y-dependence in some ways. Because there is “suggestive” trend.

A. That can be checked.

Q. For the “suggestive trends” in rapidity dependence, two-independent analysis show the same trend?

A. Yes, similar trend was observed.

C. Such detailed comparison plots should be included in the analysis note.

C. If most of Lambdas are pT>0.5 according to the acceptance plot of slide 8, the label should be "pT>0.5” instead of "pT>0.4”.

Q. Looking at mid-central data points in the right plots and comparing to BES-I results, BES-II results look smaller a bit. Is this correct?

A. Yes, that’s correct but also the results are consistent.

Q. Any plan to compare with theoretical models?

A. Haven’t considered it. Maybe for rapidity dependence but such a comparison was done in 3 GeV paper.

Slide 15

C. It would be very useful to plot Lambda and anti-Lambda separately and compare to BES-I results, before showing the difference. Not only for the consistency check but the plot can show how much the results are improved in BES-II. At least as a supplemental plot.

C. BES-II results have x-error while BES-I does not. Please minimize the errors if there is no actual meaning.

C. The data points seem to be shifted along x-axis but the shift looks different for 19.6 and 27 GeV. Better to be consistent.

Slide 17

Q. Any other possible sources for polarization difference? What should we look at this plot (B vs time)

A. Yes, there are many other sources, e.g. baryon chemical potential or freeze-out time/positions, but we start with simple motivation. This measurement provides an upper limit of B-field at freeze-out. As indicated in the B-field plot, lifetime of B-field depends on the conductivity of QGP.

Q. Feed-down correction was applied?

A. Yes for B-field calculation, but not for polarization

Q. How does the helicity efficiency affect the results?

A. The effect is largely cancelled when integrating over azimuthal angle. The generalized method also takes care of it, which is especially important at fixed-target analysis.