STAR Correlations and Fluctuations PWG

[Takafumi Niida <niida AT bnl.gov>]

Date: 2/19/2021

Participants: Samuel Heppelmann, Daniel Cebra, Sooraj Radhakrishnan, Xin Dong, Ho-San Ko, Xiofeng Luo, Barbara Trzeciak, ShinIchi Esumi, Hanna Zbroszczyk, Jiangyong Jia, Maria Zurek, Qinghua Xu, Md. Nasim, Prithwish Tribedy, Raghav Elayavalli, Hanseoul Oh, Yi Yang, Helen Caines, Takafumi Niida

Title: Measurements of Proton Higher Order Cumulants in \sqrt{s_{NN}} = 3GeV Au+Au Collisions and Implication of the QCD Critical Point

PWG: CF

PAs: Samuel Heppelmann, Yu Zhang, Daniel Cebra, Xin Dong, Xiaofeng Luo, Toshihiro Nonaka, Sooraj Radhakrishnan, Nu Xu

Target journal: PRL and PRC (short and long papers)

Proposal page: https://drupal.star.bnl.gov/STAR/blog/sheppel/protonhighermoments3gev

Presentation: https://drupal.star.bnl.gov/STAR/system/files/paper_preview_FXT3GeV_ProtonCumulants_SamuelHeppelmann.pdf

The PWGC panel previewed the paper proposal from CF PWG. The panel found that the results are interesting and important and the paper should move forward. The target journal is appropriate but also there was suggestion that PLB might be good choice for the short paper. Also there was several concerns/comments that the physics message is not clear enough for PRL and should be improved. The following points were discussed during the preview.

Fig. 1:

C. would like to see the ratio plot, e.g. data/glauber or data/unfolded single+pileup

Q. What is the fraction of pileup events in 0-5%?

A. It’s ~1.5% for 0-5% and ~0.6% in total.

Fig.2:

Q. In the bottom right plot, what is some structures seen, e.g. at p~0.8?

C. The similar structure is seen in other FXT data, and probably due to central membrane which is related to east-west alignment and timing information. t0 correction was done by Irakli but still see some effects.

Q. In the top-right plot, why do we see some structure in higher pT?

C. It reflects the structure of TOF panels, i.e. the angle of the trays and gap in backward rapidity.

Fig. 3:

Q. Higher moments decrease toward central collisions and again rise at most central bin. Why do they behave like this?

A. The dip is due to volume fluctuation and in fact the UrQMD curve shows similar trend, but the rising trend for 0-5% is not understood.

C. This would be due to broadness and non-Gaussian shape of the distribution in addition to the volume fluctuation. In that sense there are several effects in measured cumulants.

Q. How does the pileup correction affect the most 2 central bins? 

A. The pileup effect is small in 5-10% but large in 0-5% leading to the suppression of C4.

C. Centrality resolution is worse in lower energies which also leads to large volume fluctuation.

Q. What is the dominant systematic uncertainties?

A. nHitsFit for mid-peripheral collisions, while the pileup correction is dominant in central collisions

Q. Why no systematic uncertainty for higher cumulants? Supposed to be large?

A. PAs couldn’t put it because of slowness of jobs and the uncertainty for higher order would be larger.

Q. Any closure test for the pileup correction? Would like to see plots before/after the correction.

A. The closure test was done in the method paper using UrQMD. Also other colleague in STAR did it.

Fig. 4:

C. Rapidity range is different from BES-I data, which should be clarified in the figure. Also the marker for 3 GeV is not in the legend. 

Q. In caption, two rapidity ranges (-0.5<y<0, |y|<0.1) are mentioned but not in the plot. 

A. Forgot to update the caption. Based on the discussion among PAs, the point for |y|<0.1 was removed.

C. There should be some acceptance study (changing rapidity window) for BES-1. It would be nice if PAs can put the data for either |y|<0.1 or one closer to current rapidity window for a fair comparison in terms of acceptance.

Q. Why 3GeV goes down? reasonable to compare with GCE?

A. Suppression is expected due to baryon conservation.

C. but within given acceptance

Systematic uncertainty:

Q. Did PAs check if the difference from the default cut is due to statistical fluctuation or really systematic effect?

A. At this point, the statistical fluctuation may be large. Also currently the test embedding is used, but the official one will be used once the embedding production is done, which may improve the systematic uncertainty.

C. Statistical fluctuations should be treated properly and Barlow check can be used.

Q. Fig. 11: UrQMD curves are shown for this pT acceptance study, why not in rapidity window plot?

A. Didn’t think about it. PAs will consider to look.

Q. Fig. 13: This looks important plot and worth to go into the short paper.

A. PAs consider it.

C. In slide 25, 2nd bullet of physics message, the last statement sounds punchline of the paper. Would be nice if PAs can add any quantitative  comparison with model calculations to support the statement.

A. Using UrQMD, the baryon conservation was removed from the model, and then the signal goes away.

C. Maybe worth to put it in the paper.

C. Any plan if the short paper was not accepted by PRL?

A. didn’t think about that.

C. In Fig. 5, 6, since the acceptance gets wider from right to left, maybe helpful to put arrow. or perhaps clarification on x-label.