STAR Flow, Chirality and Vorticity PWG

[Takafumi Niida <niida AT bnl.gov>]

Date: 8/27/2021

Participants: Ashik Ikbal, Declan Keane, Prithwish Tribedy, Daniel 
Brandenburg, Daniel Cebra, ShinIchi Esumi, Hanna Zbroszczyk, Maria Zurek, 
Matt Posik, Md. Nasim, Qinghua Xu, Raghav Elayavalli, Hanseul Oh, Sooraj 
Radhakrishnan, Xiaofeng Luo, Rongrong Ma, Takafumi Niida

Title: Electric charge and strangeness dependent directed flow of produced 
quarks in Au+Au collisions at RHIC
PAs: Ashik Ikbal, Declan Keane, Prithwish Tribedy
Target journal: PRL
Proposal page: 
https://drupal.star.bnl.gov/STAR/blog/ashik/quark-coalescence-and-directed-flow-auau-collisions
Presentation: 
https://drupal.star.bnl.gov/STAR/system/files/pwgc_preview_aug272021_ashik.pdf

The PWGC panel previewed a paper proposal from FCV PWG. The panel found that 
the results are interesting and the paper should move forward. In the current 
form, it is still not clear what one can learn from the measurements. The 
physics message needs to be sharpen and all the assumptions made should be 
clearly stated in the paper. The proposed journal is appropriate. The 
following points were discussed.


s5: D0 v1 measurement was done at 200 GeV, not in BES-I.

s8: This could be coincidence since we only look at the two cases. 

s10: Q. Why is the difference observed even if they have same quark contents? 
They could be produced via coalescence.
A. Transported quarks have different v1 and the coalescence and sum rule are 
different things. It depends on what fraction of transported quarks is 
present in final hadrons.
C. One could estimate those fractions empirically, e.g. p-pbar ratio for 
protons, pions from delta, pair and associated production of kaons, although 
this is out of scope for this paper.

s12: To estimate the EP resolution, especially for better resolution in the 
left, one should use the formula based on Bessel function (not by using 
\sqrt(2) in the correlation).
Q. Why Res{ZDC} in 0-5% and 5-10% are almost same, while “published” data are 
not.
A. No idea. Will double check.

s13: Q. The systematic error shown here is relative uncertainty for v1?
A. No, it’s absolute value of the uncertainty on d\Delta v1/dy.

s14: Q. Are these really the first measurements of Xi and Omega v1?
A. Xi v1 in FXT may be in FXT paper. Omega would be the first measurement at 
least at STAR.
C. Please follow up to verify this. By quick research, this may be true.

s16: Invariant mass plot as Fig. 1 can be dropped. It would be better to show 
v1 extraction plot, such as v1 vs mass.
A. Xi and Omega were identified with KFP, which gives good purity. Therefore 
the invariant mass method was not used.
C. Consider to show pT dependence like the one in slide 31.

s17: Q. In the right bottom panel, why the systematic error for one of the 
data points is so large?
A. No idea. PAs will do check it again.

s19: The results are shown as a function of \Delta-q and \Delta-S but the two 
quantities on x-axis are correlated. For some cases, they are degenerated. 
Saying the slopes observed for the two axis are contradicting statement. It 
would be better to show them in one plot with another axis, which allows 
readers to identify the particle combination from the plot.
A. This plot was made to see the trend and which quantity better scale with 
d\Delta-v1/dy. PAs agree with this point and consider how to present the data.

Q. Negative v1 is seen in 200 GeV. Any comment?
A. Could be statistical fluctuations.

C. All the data points have large uncertainty. One data point in both 
energies may have ~2sigma at maximum, but the other data points touch with 
zero. A fit with constant (not linear function) could also work. The one data 
point with ~2sigma away from zero seems to basically determine the slope 
requiring the fit function crossing zero, which is also assumption. Not sure 
how significant the slope is if we don’t use one of the data points. 

C. There is issue on plotmanship in Fig. 3, which should be improved.

C. Not only the x-axis but each data points are correlated since they are 
combination of some particles.

Q. How does QCD make \Delta-S dependence?
A. Probably the term “QCD” is not good. It’s not about fundamental QCD but 
rather cross sections of s and sbar quarks and interaction with medium. This 
will be rephrased.

Q. The abstract mentions “QCD-driven effect”. What can we learn on this from 
this measurement.
A. Originally motivated to study EM effect but we find that strangeness plays 
a role.
C. \Delta-q and \Delta-S are not two separate things, how one can say if this 
is QED or QCD effect?
A. Not sure which is dominant.

Q. If we plot other combinations (not only with produced particles), how this 
plot, especially, dq and dS, looks?
A. Haven’t checked but focus on produced quarks avoiding transported quarks.

s20: Q. Why the slope is larger for 27 GeV than for 200 GeV?
A. v1 magnitude becomes larger at lower energies.

Q. Is it possible to show PHSD model curve with turned-off EMF?
A. PAs will contact the authors and see if it can be done.

Q. No error for AMPT?
A. No, it is taken from theory paper.

Q. PAs mention “new idea to show the coalescence sum rule” but they were 
discussed and presented in previous STAR PRL paper.
A. PAs will check the paper.

s31: Q. pbar and LambdaBar show similar v1(pT). They have similar mass but 
have non-zero \Delta-q. Why they are very close?
A. It should be considered based on quark mass, not on hadron mass.
C. Could be true but at least for v2 NCQ scaling is discussed based on hadron 
mass. It may depend on the combinations and assumptions.
A. u and d are assumed to be same but s is different.
C. Would be nice if it can verified. Maybe useful to plot as a function of 
mass and/or to show u-d difference.

Other comment:
Proposed figures do not include v1 for other particles. If PAs have better 
quality of data using EPD which have never been published, please include 
them as supplemental materials.