STAR GPC #355

[Yu Hu <yuhu AT lbl.gov>]

Dear Yicheng, and all,

Sorry for the late reply, please find my comments below:

Long paper:
L72. postive -> positive
L303. The Y_{bkgd} is therefore some sort of average δr/r weighted by a minv -dependent function -> The Y_{bkgd} therefore can be described by the average δr/r weighted by a minv -dependent function
L305. Repeated use “therefore” again, might want to change one of them
L312. reads a little bit confused. maybe change the second “multiplicity” to “general multiplicity”?
L446. ; this is -> , which is
L527. Why do you divide sqrt(3) here? Is that because of the 3 sources you use here? Or you are assuming a normal distribution but take 99.7% as a maximum, thus taking sqrt(3) to get 1 sigma. We should explain it clearly here.
L546. A comment on this paragraph, looks like only here you used the efficiency corrected N if I understand correctly. Need to explain why the N used in other places can use efficiency uncorrected value. Or I might be wrong, all of the N is efficiency corrected. then we might want to bring this paragraph earlier to describe how you do the efficiency correction.

Short paper:
L12. isobar data -> isobar ratio
L233. This first sentence seems not very precise. Maybe change to something like “We estimated the multiplicity dilutions caused by correlated pairs with charge dependents using pion pair multiplicities in isobar collisions. ”?
L240. maybe change the “arrive at” to “report”?
L242. “The” -> “These”
L243. add the reference after “the STAR measurements”
L244. I think the last sentence needs to specify it is Ru+Ru. Or we need to add a line after L232 say we got a comparable fraction from Zr+Zr collisions.

Minor comments to the plots. Some plots spelled out the full event and sub-event, and some used FE and SE. Maybe just use FE and SE to make them consistent?

Thank you.

Best,

Yu

On Mon, Jul 17, 2023 at 8:48 PM Prithwish Tribedy via Star-gpc-355-l <star-gpc-355-l AT lists.bnl.gov> wrote:

Dear Yicheng and all,

Here are my comments on version 14 of the short paper. 

https://drupal.star.bnl.gov/STAR/system/files/IsobarNfBkg_S_v14.pdf

The part of the paper that needs a bit of strengthening is that the short paper cannot be a condensed version of the long paper. Otherwise, the referee might suggest to merge the two papers into one and target a more specific audience of the heavy-ion community, rather than the broader physics community.

Our original focus is always to search for possible evidence of CME, and we have done an excellent job of rigorously estimating the background baseline. However, if we want to submit this to PRL, the tone of the paper should be to look for residual CME signals after accounting for the modified baseline. The background estimation is a necessary step, but it is not our physics goal – as suggested correctly by the title of the paper “Upper limit on the CME…”

I think a small tweak of rewording would do the job. My following comments are mostly about reducing technical jargons in the beginning of the paper, starting from the abstract.

Abstract:

Some minor polishing of wording is needed. It begins with a technical sentence. 

I suggest we start with a sentence about what “CME '' is and the importance of constraining the observability of CME through experiment. 

Something like (this is an example)
The chiral magnetic effect (CME) is a phenomenon that arises from the QCD anomaly in the presence of an external electromagnetic field. The experimental search for its evidence has been one of the key goals of the physics program of relativistic heavy ion collider.

We have reported the isobar (Ru+Ru, Zr+Zr) results from a blind analysis on the search for the chiral magnetic effect (CME). 

→

(make sure you keep the flow right with the previous sentence and revise like this) 

The STAR collaboration has previously presented the results of a blind analysis of isobar collisions (Ru+Ru, Zr+Zr) in the search for the chiral magnetic effect (CME).

For the following part needs to give a motivation about possible CME signal, to make it more appealing, an analysis based on a modified background baseline is not enough. We have to say the motivation is to extract the possible remaining signal, if any. 

“The Ru+Ru to Zr+Zr ratio of the CME-sensitive charge-dependent azimuthal correlator (∆γ), normalized by elliptic anisotropy (v2), is observed to be close to but systematically larger than the inverse multiplicity (N) ratio. The background baseline for the isobar ratio Y = (∆γ/v2)Ru/(∆γ/v2)Zr is naively expected to be (1/N)Ru/(1/N)Zr,”

→

“The isobar ratio (Ru/Zr) of CME-sensitive observable, $v_2$ scaled charge separation ($\Delta\gamma/v_2$), is close to but systematically larger than the inverse multiplicity ($1/N$) ratio, the naive background baseline.” 

“,however, genuine two- and three-particle correlations are expected to alter it.”

→

This indicates the potential existence of a CME signal, as well as the presence of remaining non-flow background due to two- and three- particle correlations, that is different between the isobars”

“We estimate the contributions from those correlations to Y, utilizing the STAR isobar data as well as hijing simulations”

→

“In this post-blind analysis, we estimate the contributions from those correlations to $\Delta\gamma/v_2$ ratio, utilizing the STAR isobar data as well as hijing simulations”

Main text:

The line 42-46 seems to suggest that we assumed the isobars to have the same elliptic flow because they have the same mass numbers. Then line 46 says, "in contrary to...elliptic flow differs by a few percent" - this is not accurate, we knew the flow background would be different. We have plots in BUR before the isobar run. We only overlooked the multiplicity part and that was because the shape differences were not known at that time and models didn't have the precision to predict small multiplicity differences.

The discussion from line 52-57 needs to make it explicit that the blind analysis included the v_2 part, but left out the multiplicity scaling.

Line 52:

“The multiplicity difference resulted in the isobar (Ru+Ru/Zr+Zr) ratio of the quantity ∆γ/v2

smaller than unity.” 

→

“Although the ∆γ/v2 was expected to account for the elliptic flow difference, the isobar (Ru+Ru/Zr+Zr) ratio of the quantity ∆γ/v2 was smaller than unity, due to the multiplicity difference that was not considered in the blind analysis.”

Line 53:

“If number of correlation sources is proportional to multiplicity, then isobar ratio of ∆γ/v2

would equal to that of the inverse multiplicity (N ) for pure background; the data show it is larger [24], suggesting finite CME signal [30].” 

→ 

"If the number of correlation sources is proportional to multiplicity, then the isobar ratio of ∆γ/v2 would be equal to the inverse multiplicity (N) ratio for a pure flow-driven background scenario. However, a quantitative comparison shows that the ∆γ/v2 ratio is slightly larger than the (1/N) ratio [24], indicating the potential presence of a CME signal [30]."

Line 58-60: “However, the measurement of…violated”

 This part is not clear. What is proportional to what and what is being violated ? 

Correct me here. To summarize these lines, the data larger than 1/N ratio could imply a positive CME signal, while the data lower than r ratio could imply a negative signal. However, the reasoning is not clear, because both 1/N and r are approximate proxies for naive baselines and they may not reflect the true background (or say why r is better, pair vs. single multiplicity difference). Therefore, we need to do a better job to separate the CME signal from the background, which is what we aim to do in this Letter. If this is correct, then you need to rephrase the current texts. 

Line 63: (again, here let’s emphasize the bigger goal)
“Obviously, a more rigorous assessment of the background baseline is called for, which is the subject of this Letter”

→

“In order to search for any residual signals of CME, a more rigorous evaluation of the background baseline is necessary, which is the main goal of this Letter."

Line 77:
“The background contribution can be expressed as”

→

“The background contribution to the ∆γ/v2 from intrinsic two- and three-particle correlation can be expressed as”

Line 95:
“where \deltaX= X^Ru - X^Zr and all”

→

“where δX≡ (XRu − XZr) for any X=C3p, C2p, etc., while all other quantities without "δ" refer to those in Zr+Zr.” 

Line 97:

Instead of "genuine", we could use "intrinsic" to describe the three-particle correlation, or we could explicitly define it as the correlation that is uncorrelated to global geometry, mostly arising from fragmentation of mini-jets. 

Line 103 is too long, change the first part to this:
“The isobar blind analysis [24] presented seven different measurements of ∆γ/v2; four of them used two-particle cumulants for the v2 measurement and three-particle correlators for ∆γ.”

It is probably better to introduce blind analysis “Groups” here as you introduce them on line 145 abruptly. Make sure you avoid the confusion between seven measurements vs five groups. 

You can also rephrase line 103 as:
“The blind analysis [24] had seven \Delta\gamma/v_2 measurements from four groups. We will use Group-2,3,4 results, which used two-particle cumulants for v2 and three-particle correlators for ∆γ.”

 

Line 107:
“We focus on the four cumulant measurements with the corresponding (slightly different) analysis cuts.”

→

“We focus on the four measurements using cumulant methods that only differ by slight variations of kinematic cuts.” (FACT-CHECK IF THIS IS WHAT YOU MEANT)

Line 117: HERE YOU HAVE TO SAY WHAT THIS INGREDIENT IS, REPHRASE LIKE
THIS:
“The first ingredient \delta(C2p/N)/(C2p/N) which is related to…., is primarily determined by…”

Line 119:

THERE IS A TYPO in the sentence, “as” → “and” I think. 

Line 128:
Can you please refresh my memory on where this efficiency corrected POI is used (besides Fig.1). As you know, the purpose of isobar was to reduce uncertainties due to efficiency by taking ratios, so we need to make sure we don't create any confusion. 

Line 151-153: 

I am not sure how to do this. We probably can’t refer to preliminary data. Maybe we are just “based on a separate study from STAR [36]”. 

Line 153:

"The last, common to both isobar systems, cancels in δεnf /(1 + εnf) and only the former two sources contribute.”

→
“The last source, common to both isobars, cancels in δεnf /(1 + εnf) and only the first two matter.”

Line 156 & 163:

“Full-event (FE)” and “subevent (SE)” need to be defined

Line 159:
\Delta\gamma not defined

Line 163:

“Y_bkgd is raised by this amount up towards unity”

→

“this increases Ybkgd by this amount towards”

Figure-1 caption:

“FIG. 1. Quantities corresponding to the full-event analysis of Group-3 [24]; others are similar”

→

FIG. 1. Isobar ratios of quantities for full-event with Group-3 cuts [24], consistent with other groups.

Fig.1 (left) and Fig. 2 (lower), increase the y-range to avoid overlap between texts and points. 

Line 183: 

“because it does not contain v2 nonflow” 

→

“because it largely eliminates v2 nonflow” 

Line 206: 

Refer to eq.5 for more clarity

Line 208: 

After “...consistent with these estimated baseline” add  “over most of the centrality bins” or say “...consistent within ** sigma…” 

Line 220-221: It’s not clear what you did here. You propagated the systematic error of Y_data in Y_data-Y_bkgd. Please try rephrasing. 

Line 223: 

“consistent with zero”

→

“Consistent with zero within uncertainties” 

Line 223-228:

“Comparing isobar collisions, we are…. CME signal fraction (f_CME)... 95% CL for Ru+Ru collisions” 

I think this part should be rephrased a little bit. You start by saying that since the data and the background estimates are consistent within uncertainties

“We therefore, estimate the upper-limit of the possible CME signals” 

You don’t have to “Assuming Gaussian with boundary at origin [40] (fcme ≥ 0),” I don’t think you need this in PRL, people know about this. 

Summary: 

I have some suggestions for improvement. The first line and some other parts of it are too technical. Btw, according to Eq.5 or Fig.3, you’re estimating the baseline of Y, which is not N *\Delta\gamma (line 242). When you say CME fraction, you can omit the *% of \Delta\gamma since it is redundant (.

I suggest you can rephrase it to something like this:
We reexamine the isobar ratio Y = (∆γ/v2)Ru/(∆γ/v2)Zr, which measures the charge separation due to the chiral magnetic effect (CME) in isobar collisions, and account for the background effects from multiplicity and nonflow (YOU CAN MAKE IT A BIT MORE GENERAL). We use a new method to estimate a background baseline for Y by comparing two- and three-particle correlations from STAR isobar data and HIJING simulations. The estimated baselines agree with the STAR measurements. We then set a 95% upper-limit on the CME fraction of ∼ 10% in isobar collisions at √sNN = 200 GeV at RHIC. (ANOTHER LINE ABOUT THE BIGGER IMPACT OF THIS RESULT WOULD BE EVEN BETTER)

I hope this helps.

Best,

Prithwish

Prithwish Tribedy
Physics Department, Brookhaven National Laboratory
Upton, NY 11973, USA
Phone: +1 631-344-8904, 631-344-3853
Fax: +1 631-344-4206
E-mail: prithwish2005 AT gmail.com

On Jul 17, 2023, at 5:09 PM, James Dunlop via Star-gpc-355-l <star-gpc-355-l AT lists.bnl.gov> wrote:

Dear All,  

  I have a conflict tomorrow, so we will not meet, 

but reiterating what we discussed last week:

please send comments today, and I'd like to get this process wrapped up

within the week.

Thank you,

  Jamie

On Jul 10, 2023, at 12:35 PM, James Dunlop <dunlop AT bnl.gov> wrote:

Dear All,

  Let's try to have a meeting tomorrow July 11 at 11 am.

Please let me know whether you can or cannot make it.

We do have to wrap up this work. 

--J

Dunlop, James C is inviting you to a scheduled ZoomGov meeting.

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On Jun 27, 2023, at 7:50 AM, James Dunlop <dunlop AT bnl.gov> wrote:

Dear All,
 We will not meet today (because of the analysis meeting) or next week (because
of the holiday) but please send detailed comments to the list.  It is time for the GPC to help
polishing the papers.  Given that we haven't identified any major issues with the analysis
left, we should probably aim to wrap up this GPC work by mid July.

Thanks,
 Jamie

--
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Please do not feel obligated to respond to this message outside of your work hours.
James C Dunlop Ph.: (631) 344-7781                       
Building 510A         Cell: (631)316-8153
P.O. Box 5000                      
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dunlop AT bnl.gov

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He/Him/His
Please do not feel obligated to respond to this message outside of your work hours.

James C Dunlop Ph.: (631) 344-7781                       

Building 510A         Cell: (631)316-8153

P.O. Box 5000                      

Brookhaven National Laboratory

Upton, NY 11973

dunlop AT bnl.gov

--

He/Him/His
Please do not feel obligated to respond to this message outside of your work hours.

James C Dunlop Ph.: (631) 344-7781                       

Building 510A         Cell: (631)316-8153

P.O. Box 5000                      

Brookhaven National Laboratory

Upton, NY 11973

dunlop AT bnl.gov

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