PWGC meeting 20220318
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.
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Questions and answers are following with:
- Black: We already answered in PWGC meeting
- Red: Updated answers
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[1]
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.
-> The above plot shows net-proton C1 as a function of ZDC coincidence rate. The uncorrected C1 (open circle) decreases with increasing the luminosity,
while the efficiency corrected C1 (filled black points) seems flatter than uncorrected one. The efficiency values are then updated to correct for the residual luminiosity.
C1 values with modified efficiencies are shown in cyan markers. Higher-order cumulants are also calculated by using the modified efficiencies.
[2]
Q. In slide 11, “Charged Particle Multiplicity” is corrected for efficiency?
-> No, this is uncorrected distribution usually used for centrality determination in A+A systems, but here refmult3+TOF matching is required in addition.
The label “|y|<0.5” should be removed.
[3]
Q. In Fig. 2, large fluctuation of PYTHIA is seen. Can it be improved?
-> It is due to statistical fluctuation and can be improved by increasing PYTHIA event samples.
Results using 80 million statistics were shown in the PWCG. Now we use 800 millions data.
[4]
Q. Fluctuation seems to be larger for C5/C1 compared to C6/C2. Is this something expected? Naively higher cumulants have higher statistical fluctuation.
-> One caveat here is that C5 is divided by C1 while C6 is by C2, so it's not fair to compare them directly.
[5]
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.
-> In this paper we focus on the sign change of fifth- ad sixth order cumulants driven by the crossover transition, which is not observed in PYTHIA 8 calculations as expected.
[6]
Q. In Fig. 2, the data are plotted for uncorrected multiplicity. How about for PYTHIA?
-> PYTHIA has twice larger multiplicity than the data, so it was scaled by a factor of 0.5.
[7]
Q. The uncertainty at mult~17 is somehow larger than neighbors. Why?
-> 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.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 "|y|<y_max” can be put instead of “|y|<0.5”.
[8]
Q. In slide 14, the x-axis “Charged Particle Multiplicity” is corrected for efficiency? Is this a fair comparison with Au+Au?
-> 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.
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.
In the following discussion, we define two multiplicity:
- mchTPC : Refmult3, conventional variable used for Au+Au analysis
- mchTPC+TOF : TOF matched Refmult3, new variables for p+p analysis
(2) Difference in TPC tracking efficiencies between p+p and Au+Au
The above right plots show the relationships between mchTPC+TOF and average values of mean corrected mchTPC.
(1) is performed based on following figures.
(2) is based on the following efficiecies.
[9]
Q. Then how is the TOF efficiency obtained?
-> In a data driven way but requiring EMC. See slide 20. TOF and EMC are geometrically correlated, therefore the tracks with TOF hits likely have EMC hits. This may need to be checked.
->
(1) TOF matching efficiency in 200 GeV Au+Au collisions
(2) TOF matching efficiency requiring EMC in 200GeV p+p collisions
(1)
(2)
[10]
Q. What is the main message in addition that the baseline is not one as seen in Fig. 4?
-> Since C5/C1 and C6/C2 stay positive, the thermalized QCD matter is not formed in p+p collisions.
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.
Thank you for the comments. We will discuss in the manuscript about a hint on the negative C6/C2 at high multiplicity p+p collisions.
[11]
Q. Related to the above discussion, what do PAs say if we see negative values in p+A or d+Au high multiplicity events?
-> Prediction of negative values from LQCD assumes thermalized partonic matter. Only negative C6 doesn’t necessarily mean the crossover phase transition in pp collisions.
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