STAR Correlations and Fluctuations PWG

[Takafumi Niida <niida AT bnl.gov>]

Date: 3/18/2022

Participants: Risa Nishitani, Toshihiro Nonaka, Ashish Pandav, Yu Zhang, Xiaofeng Luo, Bedanga Mohanty, ShinIchi Esumi, Daniel Cebra, Hanna Zbroszczyk, Jiangyong Jia, Maria Zurek, Matt Posik, Prithwish Tribedy, Qinghua Xu, Raghav Elayavalli, Subhash Singha, Yi Yang, Rongrong Ma, Takafumi Niida

Title: Measurements of Charged Particle Multiplicity dependence of Higher-Order Net-proton Cumulants  in 200 GeV p+p Collisions at √s = 200 GeV from RHIC

PAs: Risa Nishitani, Nu Xu, Xiaofeng Luo, Bedanga Mohanty, Toshihiro Nonaka, ShinIchi Esumi, Ashish Pandav, Ho-San Ko, Yu Zhang

Target journal: PRL

Proposal page: https://drupal.star.bnl.gov/STAR/blog/nish/Paper-proposal-200GeV-pp-collisions

Presentation: https://drupal.star.bnl.gov/STAR/system/files/PWGC_20220318_PaperProposal.pdf

The PWGC panel previewed a paper proposal from CF PWG. We found that the analysis is mature and the results are interesting, supporting that the paper should move forward. Target journal is appropriate, but the physics message/conclusions need to be polished and the careful choice of wording is needed in the conclusion. The following points were discussed.

Q. In slide 6, after requiring TOF matching, the average multiplicity becomes flat over luminosity. Is it true after correcting the efficiency? There could be a residual pile-up effect.

A. Such plots are not prepared but similar plots, for C1, should be in the analysis note. PAs will follow up.

Q. In slide 11, “Charged Particle Multiplicity” is corrected for efficiency?

A. No, this is uncorrected distribution usually used for centrality determination in A+A systems, but here refmult3+TOF matching is required in addition.

C. The label “|y|<0.5” should be removed?

Q. In Fig. 2, a large fluctuation of PYTHIA is seen. Can it be improved? 

A. It is due to statistical fluctuation and can be improved by increasing PYTHIA event samples.

Q. Fluctuation seems to be larger for C5/C1 compared to C6/C2. Is this something expected? Naively higher cumulants have higher statistical fluctuation.

A. One caveat here is that C5 is divided by C1 while C6 is by C2, so it's not fair to compare them directly.

Q. Large difference between PYTHIA and data is seen. Can we say something more about physics from the difference between the model and data? For example, by using Herwig, one could study the effect of cluster hadronization or something more.

A. PAs will think about it.

Q. In Fig. 2, the data are plotted for uncorrected multiplicity. How about for PYTHIA?

A. PYTHIA has twice larger multiplicity than the data, so it was scaled by a factor of 0.5.

Q. The uncertainty at mult~17 is somehow larger than neighbors. Why?

A. It could be due to statistical fluctuations, especially from luminosity dependence, but the Barlow test was applied to reduce the statistical fluctuation in the systematic uncertainty.

C. In slide 13, “|y|<0.5” in the right top corner should be removed since this plot shows rapidity acceptance dependence. Also, “|y|<x” on x-axis may not be so clear to general readers. Consider to rewrite it, e.g. “y_max” for x-label and you can put "|y|<y_max” instead of “|y|<0.5”.

Q. In slide 14, the x-axis “Charged Particle Multiplicity” is corrected for efficiency? Is this a fair comparison with Au+Au?

A. x-axis is not corrected for efficiency, so it’s refmult3. But for p+p data, since TOF matching is required, TOF matched multiplicity is converted to “refmult3” with lowest luminosity (~1kHz) and the relative efficiency of TOF matching efficiency between pp and AuAu is taken into account. See also slide 21.

C. TPC efficiency would be different between pp and AuAu. There could be pile-up even in such lowest luminosity events. One could also directly correct the x-axis by using TPC and TOF efficiency.

Q. Then how is the TOF efficiency obtained?

A. In a data driven way but requiring EMC. See slide 20.

C. TOF and EMC are geometrically correlated, therefore the tracks with TOF hits likely have EMC hits. This may need to be checked.

Q. What is the main message in addition that the baseline is not one as seen in Fig. 4?

A. Since C5/C1 and C6/C2 stay positive, the thermalized QCD matter is not formed in p+p collisions.

C. With current data, we understand that. But the data show the decreasing trend toward higher multiplicity. If we have more statistics and can measure the cumulants in higher multiplicity bins or with pp data from 500 GeV, it could be negative. So the wording should be carefully chosen. Later or at the LHC they may claim something different.

Q. Related to the above discussion, what do PAs say if we see negative values in p+A or d+Au high multiplicity events?

C. Prediction of negative values from LQCD assumes thermalized partonic matter. Only negative C6 doesn’t necessarily mean the crossover phase transition in pp collisions.