STAR Flow, Chirality and Vorticity PWG

[Takafumi Niida <niida AT bnl.gov>]

Date: 4/9/2021

Participants: Chunjian Zhang, Shengli Huang, Jiangyong Jia, Barbara Trzeciak, Daniel Brandenburg, Daniel Cebra,  ShinIchi Esumi, Hanna Zbroszczyk, Maria Zurek, Matt Posik, Md Nasim, Prithwish Tribedy, Qinghua Xu, Raghav Elayavalli, Hanseul Oh, Sooraj Radhakrishnan, Yi Yang, Xiaofeng Luo, Helen Caines, Rongrong Ma, Takafumi Niida

Title: Probing quadrupole deformation of uranium in relativistic nuclei collisions

PWG: FCV

PAs: Chunjian Zhang, Shengli Huang, Jiangyong Jia, Niseem Magdy, Roy Lacey

Target journal: PRL

Proposal page: https://drupal.star.bnl.gov/STAR/blog/chunjian/probing-quadrupole-deformation-uranium-relativistic-nuclear-collisions

Presentation: https://drupal.star.bnl.gov/STAR/system/files/PWGC_review_0409.pdf

The PWGC panel previewed the paper proposal from FCV PWG. The panel agreed that the analysis is mature, results are interesting, and the paper should move forward. The panel thinks that the target journal is appropriate but the physics conclusions should be sharpened more to meet PRL criteria. PAs mentioned other possibility of journal which can be discussed with GPC. The following points were discussed during the preview.

Q. s5: How is “R” defined?

A. R is overlap area in transverse plane and is defined as the root mean square of x and y.

Q. s10: Why Vz is included in the systematics? The physics wouldn’t depend on vz.

A. Acceptance (rapidity coverage) changes with Vz. The effect is small but included to be conservative.

Figure 1:

Q. What does the negative covariance mean in high multiplicity U+U?

A. Anti-correlation between v2 and mean pT, due to deformation.

C. Marker colors used here are opposite to those in Fig. 3. Please use the same color for the same system.

Figure 2:

Q. Not only in peripheral but also in central events, there seems to be some difference between the methods. Since the data in central events are used to constrain \beta_2 and there is no non-flow in models, this difference should be considered as systematic uncertainty when constraining the \beta_2 parameter.

A. The difference between the methods at higher multiplicity would be due to decorrelation, which is not in hydro calculation. PAs will add the explanation and discuss the effect. 

Figure 3:

Q. What does negative values of models in low multiplicity mean in Fig. 3?

A. It would be due to the lack of or different physics in peripheral collisions, such as no final state evolution and initial momentum anisotropy. Theorist also acknowledged the peripheral results may not be reliable.

Q. If you take Glauber model, i.e. eccentricity and 1/R, do you see expected correlation.

A. Yes.

Q. Why the models don’t go to higher multiplicity as seen in data?

A. In the model calculations, fixed impact parameter was used.

Q. In AuAu, one may naively expect there may be negative correlation in low multiplicity but the Pearson coefficient is always positive.

A. non-flow and initial momentum correlation give positive correlation. Also it’s true that behavior in peripheral is not fully understood. If you compare with UU, UU is alway lower than AuAu, meaning that it is influenced by deformation all centralities.

C. There is confusion in colors of the bands. Better to use the same color for the same \beta_2 among different models.

C. Raw Nch is used in x-axis. For fair comparison with models, centrality is better or Nch needs to be corrected for efficiency.

Figure 4:

Q. In the right panel, how did you get the x-value (\beta_2) for the data?

A. For U+U, the value was taken from low energy nuclear experiment (~0.28). For Au+Au, the value was taken from theoretical value (which is negative but the experimental value shows positive value). It’s not clear if the beta_2 obtained at low energy experiment based on rotor model is same as that at high energy heavy-ion collisions. This measurement provides different way to measure it.

C. Still the plot is confusing. The data could be shown with band across all beta_2 region. Crossing region with models gives us constrained beta_2 range. It might be useful to make it clear what value in low energy experiment. Basically all information are in Fig. 3, not sure the right plot is needed.

A. Most central data is more sensitive to deformation parameter as one can see in Fig. 3. Here we show only the central data to give quantitative estimate of the deformation parameter.

Q. What does the model band width in y-direction mean?

A. Statistical error of the model calculations.

C. Please consider to change the color or style of bands in the left or right panels. It’s confusing because of same color, but one in left show systematic uncertainty and the other in right show models.

Q. Looking at panel-(b) in Fig.4 and panel-(e) in Fig. 3, the AMPT value for \beta_2 looks different. Why?

A. Centrality bin width is different. 0-1% in Fig. 4 and 0-5% in Fig. 3.

C. Should be clarified to avoid the same confusion.

Q. Is it worth to show AMPT v2 and v3 in the left panels?

A. The plots get busy. Also, AMPT doesn’t describe the mean pT fluctuation. This will be pointed out in the paper.

Q. What parameter was used  for TRENTo in the left panels?

A. 0.28 for U+U. PAs will make it clear in the figure.

Q. How does the isobar look like?

A. Initial study started by Shengli. One could constrain even higher-order deformation parameter like \beta_3, which is difficult to measure in low energy experiment. If we use well known species for calibration, then one can better constrain those parameters. We are also looking at mean pT fluctuation, which may be more sensitive to deformation.