STAR HardProbes PWG

[Barbara Trzeciak <barbara.trzeciak AT gmail.com>]

Hi Yuanjing,

please find my comments to the new paper draft below.

Cheers,

Barbara

Abstract:

- =200 GeV - missing space = 200 GeV

- "The measured e HF v2 at 54.4 GeV is also consistent with the expectation of their parent charm hadron v2 following number-of-constituent-quark scaling as other light and strange flavor hadrons." - I wouldn't say it's so consistent at low pT. I would remove this from the abstract as this requires more explanation, and rather have discussion on the scaling in the results part. 

- "This hints charm quark might be not fully thermalized at 27 GeV." : here the same, I would rather say about less strong interactions than make conclusions about the charm quark thermalization. Moreover, the results suggest that charm quarks most probably are not fully thermalized at this energy. 

- In general, I would rather say in the abstract that the charm quarks interact strongly with the surrounding medium at 54.4 GeV, similarly as at 200 GeV. While at 27 GeV the data indicate that the interactions might be less strong. Conclusions regarding the charm quark thermalization should be rather drawn from model comparison, and I don't think make this conclusion based on the results here - we don't have a comparison to e.g. a hydro model where charm quark achieved thermal equilibrium.

Results: 

- L217: event-plane method at 54.4 GeV: you have the non-flow estimation for both 54.4 and 27 GeV

- L218: energies, 200 -> energies: 200, and there was also measurement at 39

- L219-220: uncertainties on the 200 GeV are smaller. Don't you mean that the 54.4 result is more precise than the previous results at 62.4 and 39 GeV ?

- L234: "indicating that charm quarks start to deviate from local thermal equilibrium": I don't think we have a prove that charm quarks reach thermal equilibrium at higher energies, so it's hard to say that they deviate from it at 27 GeV. You can just say that they interact less strongly with the medium.

- L237: you should write that TAMU prediction is for 62 Gev

- L240-241: that's in general true, but keep in mind that it's electron pT and electrons may come from D meson decays where D meson has larger pT and radiative energy loss is not so small in that range. So it might not be true that the radiative en. loss contribution is negligible here. Adding the radiative energy loss to the model could potentially increase the predicted v2 even in this electron pT region.

What you claim here is on the parton (or D) pT level, I think that should be clarified . Results for electrons are more tricky to be interpreted - you can see in fig. 81 in your AN that the distribution of e vs D pT is quite broad. 

- Fig. 7: colouring highlights the 200 GeV results - I would use black for 200 GeV and then e.g. blue for 54.4 GeV. 

- L257-259: these two sentences seem to be contradictory at the first look. Maybe something like:  Both TAMU and PHSD calculations are lower than the measured central values. However, considering the non-flow contribution, and the statistical and systematic uncertainties they are consistent with the data at pT > 1 GeV/c.

- Actually, how far from the predictions are the first two points taking into account the stat. and sys. unc. ?

- L260-261: at 200 GeV both PHSD and TAMU describe the data quite well. The predictions are below the central values but not so significantly, and the claim in the D0 v2 paper is that they are consistent  - I think that the statement that they underpredict the D0 v2 at 200 GeV for pT > 2.5 GeV/c is too strong. 

- It's been some time since the first version of the paper draft - do you know if we could get TAMU predictions at 54.4 GeV, and maybe the predictions of both models at 27 GeV ? Also, other model calculations would be useful, especially if you want to make conclusions regarding the charm quark thermalization. 

- L262: The eHF momentum differ -> The eHF momentum differs

- L262-263: The eHF momentum differ from its parent charm/bottom hadron momentum due to the decay kinematics. - you mention here charm and bottom harons, however in the following lines there's nothing mentioned about B contribution

- L262-274 (ncq scaling): you don't write that D0 spectra are actually for 62.4 GeV. In general, there are some caveats to these simulated D v2, so I wouldn't draw too strong conclusions based on this. 

- Fig. 8 caption: remove "The D → e decay kinematics are simulated in PYTHIA." - description is in the text

- L275, 284: There are no K and D0 on the plot

- L285, 286: while deviates from that of π + at low energies. - these deviations are only for two points and not significant: phi is lower than pi at 11 GeV with 1.2 sigma and e^HF is lower than phi at 27 GeV at 1.85 sigma level. - these numbers should be given in the paper

- L286: The v2 of K+ 286 also keep consistent with that of - there are no K on the plot

- L287-289: With decreasing collision energy, heavier mass particles v2 drops faster than lighter ones - this faster drop is not significant

- L289-290:  which suggests there is a change in the QCD medium properties with decreasing energy. - this is a very general statement, can you be more specific here. 

- I personally think that this figure brings almost nothing, the drop that we see is not significant. If other covneners think it's worth keeping, this can be discussed further in GPC.

- Fig. 9: there are no D0 points - you have it in the legend.

- All plots: 200 GeV -> \sqrt{s_NN} = 200 GeV, same for 54.4, 27 GeV

Summary

- L297: Several transport -> TAMU and PHSD transport

- You emphasise in the summary the comparison to NCQ scaled v2 -> I think there are many caveats here, and as I commented on the abstract, based on similarity of v2 at 54.4 and 200 GeV and comparison to models at pT > 1 GeV/c, I would rather say about strong charm quark interactions with medium.

- L305- 307: The energy dependence of measured v2 from various particles shows an indication of quark-mass dependence. - I would drop in from the summary.

On Thu, Jun 9, 2022 at 9:15 AM Yi Yang <yiyang AT ncku.edu.tw> wrote:

Hi Yuanjing,

Since we are moving to the new mailing list, I copy all the discussions (your newly released note and draft + Barbara's latest comments) to here. 

**********  From Yuanjing (May 11)  ********** 

Dear convenors,

Again, thank you for your valuable comments. We have updated our 
notes/paper draft and prepared responses to your comments. Please find 
the details below. We would like to get your sign-off and move on to 
GPC.

Paper draft: 
https://drupal.star.bnl.gov/STAR/system/files/Heavy_Flavor_Electron_v2_at_27_and_54_4_Au_Au_Collisions_May10.pdf

Note: 
https://drupal.star.bnl.gov/STAR/system/files/NPE_v2_at_Au_Au_27_and_54_4_GeV_analysis_note_May10.pdf

Website: https://www.star.bnl.gov/protected/heavy/jiyj/NPEweb

Response to convenors:
Comments to Note:
response to Sooraj: 
https://drupal.star.bnl.gov/STAR/system/files/Sooraj_NPE_v2_note_May10.pdf
response to Barbara: 
https://drupal.star.bnl.gov/STAR/system/files/Barbara_NPE_v2_note_May10.pdf
response to Yi: 
https://drupal.star.bnl.gov/STAR/system/files/Yang_NPE_v2_note_May10.pdf
Comments to Paper:
response to Barbara: 
https://drupal.star.bnl.gov/STAR/system/files/Barbara_NPE_v2_paper_May10.pdf
response to Yi: 
https://drupal.star.bnl.gov/STAR/system/files/Yi_yang_NPE_v2_paper_May10.pdf

Best
Yuanjing

**********  From Barbara  (May 29) **********  

Hi Yuanjing,

thanks for the answers and the new version of the AN and the draft.

I have a few remaining comments, please see below. And I will send my comment to the paper draft soon.

Cheers,

Barbara

Replies:

(18) - Have you considered relaxation of the mean and width parameters for the sys. unc. estimation? 

- In systematic uncertainty estimation, we also directly take the normalized nSigmaE histograms, instead of gaussian functions, as the templates to carry out purity fitting. So the uncertainties from the description of the particles’ nSigmaE shape, including mean and width, will be taken into account. And considering the statistics under most of the momentum bins, uncertainty from mean and width are quite small. Also when carrying out template fitting to extract purity, we have already had 5 free parameters (particle yields), so it is not suitable to add more free parameters

Yes, but at very low pT the sys. unc. on the purity are not even visible with the current variations. Maybe then, even if the means and withs from the fits to the pure hadron and electron samples have small uncertainties, they will give a not negligible effect compared to the other variations that you use currently for the sys. unc. estimation. My point is not to relax the mean and the sigma fully, but from your fits to the pure samples you get values that have some uncertainties, you can then put limits on the mean and width in the total fit to e.g. mean +/- 3unc. Also, up to 0.33 the merged pions don't fit so well in the total fit. 

AN: 

- nSigmaE fits for 27 GeV: here I spotted that in some bins the electron fit is taken over by hadrons. Between momentum of 1 and ~1.1 GeV/c and then in the 0.53-0.55 GeV/c range the electron gaussian is replaced by the Kaon gaussian. This is probably because you don't constrain the yields for 27 GeV and shouldn't change your results (I think all of these cases are in the excluded regions), but still it would be good to update it for future. And I think it's better to have consistent methods for 54 and 27 GeV.

- Purity estimation: what is the reason to take histograms for pions instead of constraints based on the gaussian fits to these distributions? The gaussian shape looks to describe the pion distribution well. Taking this variation as the sys. unc. introduces quite large unc. at higher pT where it seems to me that it might be driven by the statistical fluctuation in the pion histograms.  For the cases where you use histograms for pions in the total fit, could you please add stat. unc. on the purity distribution ? I'm making this point also because the purity for 54 GeV at 2-2.5 GeV/c is larger than 95% and if the unc. were smaller there, this range could be usable for the v2 calculation. This drop is not visible for 27 GeV when using histogram when you use wider bins, so I wonder if this sys. for 54 GeV is not driven by the statistical fluctuations in the pion sample - you don't have many entries in the tails of your pion distribution and then when you normalise it for the total fit, these fluctuations are enhanced. 

- L366: missing figure number

- L433: missing figure number

- "The 𝑁𝑐𝑜𝑙𝑙 in Au+Au 39 and 62.4 GeV are also taken from PHENIX direct photon paper [20] and PhD thesis on this measurement [34]." - why do you take Ncoll from the PHENIX paper, not from independent Glauber calculations ?

- Fig. 20: are these plots for 62. 4 GeV ? If so, it's missliding to label them "Au+Au 54.4 GeV", I would change it to "Au+Au 62.4 GeV" and explain in the text that these spectra are used for the 54.4 GeV analysis.

- Fig.26 and 27: you've changed binning for some of the sys. unc. sources, which could be fine, but a bit confusing because different sources now have different binning and it's also not clear how then the total sys. unc. in the fine binning is obtained. I would use the same binning for all the sources, it should correspond to the binning that you have for the reconstruction efficiency.

Also, there is no unc. for the lowest pT bin for nHitsFit.

L483: uncertainty for is given in Fig. 26. Figure 29 shows - there's a missing word "for ... is", also Fig.26 and 27, and Figures 28 and 29 show.

With the reconstruction efficiency obtained from the embedding simulation -> is it the same as the combined reconstruction efficiency that you describe ? What's the difference between efficiencies in Fig.28,29 and fig. 37?

L578: flatten Fig. ??(a) -> missing figure number

Fig. 63, 64: why for 54.4 you have Npho stat (data) and Npho stat (embed) while for 27 GeV there's only one contribution: Npho stat, is it only from data ? And why the Npho stat (embed) for 54.4 has quite a big contribution, larger than sys. unc. in some bins ? 

Fig. 80: second HFe v2 point at 27 GeV is missing.

**********  From Yi (June 8) **********  

Dear Yuanjing,

Thanks a lot for the updated note, paper draft, and replies. 

I needed to go through them again since it was almost a year ago and I forgot some details.

I think the note and draft are ready to move forward. 

I have some very minor comments for your consideration: 

  - Analysis note: 

    - The replies are good to me.

    - L366: Fig. ?? --> Fig. 9

    - L433: Fig. ?? --> Fig. 14

  

  - Paper draft: 

    - Fig 2 (a): consider to use points for data (not black histogram) 

    - L114: you use "opposite-sign" and "unlike-sign" at the same time, it would be good to pick one of them. 

    - L120: similar for "same-sign" and "like-sign".

    - L131: Au+Au and p+p --> The Au+Au and p+p 

    

Cheers,

Yi

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Yi Yang, Associate Professor
Department of Physics
National Cheng Kung University
Tainan, 701 Taiwan
E-Mail: yiyang AT ncku.edu.tw
Tel: +886-6-2757575 ext.65237
Fax: +886-6-2747995
Group Web: http://phys.ncku.edu.tw/~yiyang
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