STAR Correlations and Fluctuations PWG

[傅川 <fuchuan AT impcas.ac.cn>]

Hi Grigory,

Thank you for your comments.

Please find the response to your comments below (page 14):

https://drupal.star.bnl.gov/STAR/system/files/2-Comments-and-replies-for-PWG.pdf

 

Regards,

Chuan for the PAs

-----原始邮件-----
发件人:"Grigory Nigmatkulov" <nigmatkulov AT gmail.com>
发送时间:2023-07-11 14:24:18 (星期二)
收件人: "傅川" <fuchuan AT impcas.ac.cn>
抄送: star-cf-l AT lists.bnl.gov
主题: Re: Fw:Re: [Star-cf-l] CF PWG meeting, June 1st, 2023, 9:00 (BNL time)

Dear Chuan, PAs, and All,

Thank you very much for your reply. Please find some additional comments to your nice

notes and analysis note. I'll probably send more comments when I have time to read the notes

more carefully.

>The p-\Lambda, p-\Xi pairs from same and mixed events are used to calculate k* distributions, the
>contribution to same event and mixed events from splitted/merged proton tracks are considered
>to be same, and is canceled. The splitting and merging effects will not effect p-\Lambda, p-\Xi CFs.

This is a very strong statement and should be demonstrated. In the past, the effect was NOT negligible.

Please show the distributions for primary (a.k.a. selected proton track) and daughter proton 

from Lambda decays. Please demonstrate similar effects from negatively charged daughter tracks.

To demonstrate the effects of splitting and merging, please plot (at least) angular correlations between

those (in terms of delta eta vs. delta phi* or spatial separation between hits) as well

as track splitting variables (NOT variation of nHitsFit, see the comments below).

>For p-p CF, the number of TPC hits (nHitsFit) is used to improve the track quality and remove the
>splitted proton tracks. The merged proton tracks are removed by using \delta \phi and \delta
>\theta cuts. More discussions about splitting and merging effects for p-p CF can be found in the
>note: slide 7-9

1. Variation of nHitsFit is NOT a proper choice of removing track-splitting effect that appears in correlation femtoscopy.

nHitsFit and nHitsFit/nHitsPossible will help to remove tracks that were splitted at the central membrane

and NOT the effect of close tracks. Moreover, requiring of nHitsFit to be large may very likely distort

correlation function because nHits and track momentum are correlated. In case of a large number of hits

the low momentum track will be cutted off. Because of that, the Splitting Level variable was developed some time ago.

For the reference see:

https://arxiv.org/pdf/nucl-ex/0411036.pdf

In other words, applying cut (from slide 7): NhitsFit (N1 and N2) >= (45+1)/2

is not convincing.

2. As for the track-merging effect. Slide 8 (for p-p correlations) is not convincing as well.

First point here is that delta phi is almost insensitive to the track merging. One should use at least

delta phi* (azimuthal angle at the given radius). Please plot 2D distribution of  delta eta vs. delta phi* with 

positive and negative values for each variable and demonstrate the effect.

3. Momentum smearing. Please plot momentum distributions (spectra) for particles from the experiment and UrQMD.

If spectra are not the same then using UrQMD to correct for the momentum resolution should not be used

and a data-driven approach must be considered. This point is valid for all three particle combinations

under the study (pp, pLambda and pXi).

>Yes, in purity correction we consider the CF shape from sidebands:
>p-\Lambda: In analysis note section 2.7, we showed the CFs of \Lambda selected in the sideband,
>and we used these sideband correlation function to do the purity correction
>p-\Xi: section 4.2.2

Thank you for these nice figures. Now I'm confused. How do you explain and correct for non-unity correlations

in case of p-\Lambda (Fig. 16)? This is very unlikely to be corrected by the purity correction factor.

In other words, you have some correlations from the background that imitates the signal. How do cross check

that these correlations disappear after the correction? Please demonstrate it.

As for the Fig. 8 from p-\Xi correlation function for sidebands. Why did you use such a complicated equation

to fit the CF ([0] + [1]*x + exp([2] + [3]*x)? Please demonstrate fits and its parameters for fit with constant and linear functions.

>For p-p CF, we do not apply this correction since the proton purity > 98% for each momentum bin,
>and the most part of protons we used are from p [0.4, 1.0] GeV/c, the purity > 99%. In addition,
>we change the nSigma_proton cut value to be 1.5, and the CF difference from default and varied
>cut value is included into the systematic uncertainty.

I do not understand the statement. If one varies the n\sigma cut for systematics then you need to demonstrate

that after the purity correction results will be different. Simple variation of n\sigma changes the contamination

from other particles and affects the signal as well as a background. One should prove that this variation

carries out the systematic uncertainty.

Another odd source of systematic uncertainty is momentum resolution correction. Usually one compares the values

before and after correction and may use those as a systematic uncertainty. The variation of the momentum smearing width

is not clear. This should be discussed.

>We keep the p-Xi CF since it is the first measurement in the high baryon density region, and also it
>will be a good reference for future 2 B 3 GeV data

slide 9 (Fig. 3 for the paper proposal). CF for p-\Xi. Does UrQMD CF include statistical uncertainties?

For all analysis. Could you please tell how you calculate correlation functions for UrQMD?

Do you calculate CFs, in Au+Au CMS or in the Lab frame? If the latter, how do you calculate the Lab frame?

>The f0 and d0 measurements from STAR/ALICE/CEBAF for p-p pair and p-Lambda pair are scarce.
>The f0 and d0 of p-p pair showed in 200 GeV nature paper is obtained based on the phase-shift
>data from low energy scattering experiment. In heavy ion collision, the f0 and d0 measurement is
>very scarce.
>We did not add other f0 and d0 measurements in fig. 4

I do not understand the term scarce at all. Here are some results from STAR and CEBAF:
https://arxiv.org/pdf/nucl-ex/0511003.pdf

They are of high precision. Here are some recent results from ALICE:
https://arxiv.org/pdf/1805.12455.pdf

Those are of high precision too.

Do you have any expectation that final state interaction parameters for high baryon density regime will be different

from high energy collisions? We must include those and compare the new results with those obtained

by others.

>We use LL model to fit data and extract the Gaussian source size rG from data and FSI parameters
>f0 and d0. The UrQMD+CRAB model is also used to fit data and extract the corresponding source
>size (RMS). In addition, we use LL model to fit the CF from UrQMD+CRAB model and extract the
>source size rG from UrQMD, in order to make a comparison with RMS extracted from UrQMD.

I do underant the first part, where you fit the LL model to data.

I do not understand the second part. What is the point of making any of it? Do the final state interaction parameters

from UrQMD (that you include as a weight) agree with those extracted from the fit? Please demonstrate it.

Cheers,

Grigory