STAR Flow, Chirality and Vorticity PWG

[Chunjian Zhang <czhang.phys AT gmail.com>]

Hi All,

Just want to point out that we had some nonflow and physics studies in pythia/hijing/ampt models regarding the observables in 2102.05200, 2112.03397, 2105.05713.

thanks,

Chunjian 

On Thu, Jan 25, 2024 at 11:06 AM Sooraj Radhakrishnan via Star-fcv-l <star-fcv-l AT lists.bnl.gov> wrote:

Hi All,

   Sorry I could not join the meeting yesterday during the discussions. So if this was discussed, apologies 

HIJING could be overestimating non-flow as it does not have medium evolution/thermalization. For non-flow, the medium evolution would be contributing a decorrelating and diluting effect. 

Since AMPT uses HIJING as input, it would be good to see comparison of AMPT and HIJING. Looking at two particle delta eta - delta phi correlations should tell what the impact is 

Best,

Sooraj

On Thu, Jan 25, 2024 at 5:33 AM Jiangyong Jia via Star-fcv-l <star-fcv-l AT lists.bnl.gov> wrote:

Hi, Sergei,

1) We have provided an upper limit estimate based on 1/Nch scaling assuming most peripheral bin is all non-flow for v2 (1/Nch scaling is seen in HIJING and also roughly seen in Isobar, though latter show jet quenching effects).  For fixed pT, u and Q, <UQ*>are the same, so it is what is done in both studies.

We just say the effect is only a few percents in the ratio of UU/AuAu, since only the DIFFERENCE matters.

2) Particles after jets quenching is correlated with geometry, i.e. v2 \propto e2 as we know.

3) When jet are quenched and become medium particles. They will also flow and become correlated with participant plane. 

What we mean is experimentally you can not precisely define non-flow in central AA collisions (ALICE for Xe+Xe/Pb+Pb did not do non-flow subtraction).

4) "surface emission" is not flow.  By that we mean correlation between fragments within unmodified jets is non-flow. But of cause the leading particle from surface emission will have a non-zero v2 and is correlated with e2. 

5) hot spots" effect (mentioned in relation to 2-particle CF) is non-flow (again I could have misunderstood it) -- this is incorrect.

We agree correlation arising from hot spot at initial condition is flow.

Jiangyong

On 1/24/24 11:21 PM, Sergei Voloshin via Star-fcv-l wrote:

Hi all,

There was a rather interesting discussion at today's meeting (thanks to Fuqiang and Jianyong for preparing the presentations!), though I had to leave after 2.5 hours...  I have a few comments (not in any particular order) below.

- The discussion could be much more productive, if somebody would chair the session. It could be better if during the presentation only short questions  were allowed, and longer discussions delayed for later. Especially when it is clear that people agree to disagree (but not only).

- I agree with Fuqiang that a clear estimate of nonflow should be made, e.g.for reported v_2, but for other quantities as well.

- For the nonlfow estimate HIJING can be used, though the data might be preferable. From my point of view, using pp data would be the best if used <uQ*>, AA-pp method. One can use peripheral collisions, as tried by Jianyong, including even the scaling presented (though again much better would be <UQ*> approach). Even better would be to compare different estimates.

- The statements were made that it is not really possible to define nonflow. Very strange - what do we report then? Let us define what we want to measure, e.g. the correlation with the participant plane (another possibility would be the correlation with the reaction plane), then what is extra in e.g. measurements with two-particle correlations would be non-flow. 

-  Again, if we report v_2, let us define what we mean by that and report a realistic estimate of "non-flow" in that measurement. The values around a few percent for (ultra)central collisions look very unrealistic, to say the least.

- The paper mentions "standard" and subevent methods, but neither is defined (unless I missed that).

- The effect of nonflow for AuAu and UU collisions is likely rather similar, and the effect on the ratios should be calculated taking this into account. (This is probably what Jianyong meant by "difference" in nonflow)

- Statement that "surface emission" is not flow, is incorrect (unless I misunderstood what was said) at least for the definition of flow I used above,  but it indeed depends on the definition of flow.

- The statement that quenching reduces nonflow is (likely) incorrect, in fact it could increase it. The result of quenching is that instead of one high(er) pt particle we have several low pt. But those still have azimuthal angle strongly correlated with "jet" - then the contribution to nonflow would only increase. 

 

- "hot spots" effect (mentioned in relation to 2-particle CF) is non-flow (again I could have misunderstood it) -- this is incorrect. More exactly it again depends on the definition of flow used. If it is correlations relative to the reaction plane, the statement is correct, if it is relative to the participant plane, the hot spots effect is already included in geometry/flow plane fluctuations.

Best regards,

Sergei

-

--

======================================
 Sergei A. Voloshin    

 Distinguished Professor, 

 Department of Physics and Astronomy,

 Wayne State University,
 666 W. Hancock,  Detroit,  Michigan,  48201
 e-mail: sergei.voloshin AT wayne.edu
======================================

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--

Sooraj Radhakrishnan

Research Scientist,

Department of Physics

Kent State University

Kent, OH 44243

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

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