STAR Flow, Chirality and Vorticity PWG

[Sooraj Radhakrishnan <skradhakrishnan AT lbl.gov>]

Date: 05/17/2024

Participants: Achyut Khanal, Sergei A. Voloshin, Takafumi Niida, Jae Nam, Ting Ling, Hanna Zbroszczyk, Prithwish Tribedy, Isaac Mooney, Nihar Sahoo, Yi Yang, Zaochen Ye, Barbara Trzeciak, Shinichi Esumi, Sooraj Radhakrishnan

Title: Longitudinal flow-plane decorrelation with multiple-plane  cumulants from STAR

PAs: Achyut Khanal, Takafumi Niida, Sergei A. Voloshin

Target journal: Phys. Rev. C

Proposal page: https://drupal.star.bnl.gov/STAR/pwg/common/policies/pwgc-preview-requirements 

Presentation: https://drupal.star.bnl.gov/STAR/system/files/pwgcPreviewDecoMay17.pdf 

The PWGC panel previewed a paper proposal from FCV PWG. The panel found that the analysis is mature and results are important and interesting, and the paper should move forward. The journal choice was also found to be appropriate. The following points were discussed.

Q: On S3, what are sources of the different kinds of twist shapes and random walk decorrelation?

A:  The goal is to see what kind of twist/decorrelation is there in the data. The nature of particle production from the participants could cause these different kind of effects. For example, If the rapidity distribution of particles from each participant is small, would cause a random walk like decorrelation 

Q: S12 (Figure 3), is this the CMS correlator?

A: Yes

Q: S13, what is the particle level approach? What is the difference you are attempting to evaluate with this approach as opposed to the Q vector approach

A:  Particle level approach uses particle azimuthal angles in the TPC directly to correlate with the EPD event planes. This is a cross-check. Similar to different sensitivity for EP method and scalar product method for analyses 

Q: S13, The product of sine terms before the cumulant subtraction show clear negative values. The EPs are also well separated. Why can't you conclude a twist from this? Why do you need the cumulant approach?

A: We should subtract the lower order correlations to evaluate the true decorrelation. 

Q: Have you done any simulation studies, like with AMPT or Glauber, to see how your cumulants are correlated to the participant plane decorrelations in the models?

A: We have not done model studies with AMPT etc. The paper referenced has done. We have also done studies by putting in different twists to the EPs measured in data and applied the cumulant method to calculate the twist. We could recover the input. See S47

Q: You see a small C-shape, within one sigma. Why do you not add it in the conclusion. Is it because of what you say on S20, that you can't distinguish the C-shape twist from the decorrelation expected between the mid-rapidity EPs and the one evaluated from spectators?

A:  Yes. The signal is only one sigma. Also we cannot distinguish between the two effects

Q: You see large signal with the r2 observable and also difference between the two isobar species. But your cumulant correlator does not capture either. What does this mean?

A: This would imply that the decorrelation is random walk like. Also any non-flow contributions will be removed 

Q: What is the decorrelation between the participant planes from forward and backward going nuclei from models?

A: We haven't evaluated this, but will calculate 

Q: Do you expect to have a larger signal in a larger system, Au+Au as opposed to isobar? Do you expect to have energy dependence, larger dependence at lower energy?

A: This is an interesting question. For lower energies, the limitation is our acceptance relative to beam rapidity decreasing 

Q: Can you do these measurements with the first order EP?

A: We could use the first order EPs as the EPs in forward regions 

--

Sooraj Radhakrishnan

Research Scientist,

Department of Physics

Kent State University

Kent, OH 44242

Physicist Postdoctoral Affiliate

Nuclear Science Division
Lawrence Berkeley National Lab
MS70R0319, One Cyclotron Road

Berkeley, CA 94720

Ph: 510-495-2473

Email: skradhakrishnan AT lbl.gov