STAR HardProbes PWG

[Rosi Reed <rosijreed AT lehigh.edu>]

Hi Rongrong,

I agree that they are both looking at the correlation of the object of interest with the event plane.  How we construct v2 would be the same regardless, it's really just a Fourier coefficient.  I also agree that non-flow is the v2 signal originating from pQCD based geometry correlations - i.e. jet-initiated correlation.  So he is entirely looking at nonflow (well, at least at the higher pT).  We use the EPD in order to ensure that the presence of the jets don't bias the determination of the event plane - essentially one needs some sort of rapidity gap so that the jet is really being compared to the underlying geometry and not influenced by some jet-based autocorrelation.  (If you recall, Alice Ohlson did a study on this: https://arxiv.org/abs/1205.1172). 

When using the EPD to determined the event plane, there still will be a contribution at mid-rapidity from jets - it doesn't remove that, it only means you can measure the different quantities such that they aren't causally connected.

In any case, I think we agree that v2_obs works in both cases, and I do think understanding the results will be the work of the next few months.  (For instance, there's the question of what happens if we use the EPD for the centrality, since in a small system the mid-rapidity determination is more likely to be influenced by non-flow.)

Cheers,

Rosi

On Tue, Aug 29, 2023 at 9:19 AM Ma, Rongrong <marr AT bnl.gov> wrote:

Hello Rosi

Yes, Shengli and Tristan are looking at different physics, but they use the same observable, i.e. correlation of object of interest with event plane, not with jets. Shengli focuses on low pt, while Tristan focuses on high pt. I think non-flow refers to the v2 signal originating from jet-initiated correlation. What Tristan wants to measure is correlation of jet or high-pt track with event plane, not those generated by jets themselves. Isn't the reason to use EPD is to suppress non-flow? Otherwise, we can use TPC to reconstructed event plane, which has a higher resolution. Shengli's results show that non-flow contribution is still very significant at high pt even if one uses EPD, which could be due to away-side jets, I suppose. 

Best

Rongrong 

On Aug 29, 2023, at 9:08 AM, Rosi Reed <rosijreed AT lehigh.edu> wrote:

Hi Rongrong,

I'm a little confused, the greatest contribution to non-flow comes from jets, which is what Tristan is looking at.  So we wouldn't want to subtract non-flow from his results, basically the non-flow is his results.  There is perhaps a question of how much flow is contaminating his results at 5 GeV.  I think writing v_2^obs for the tracks is fine - the main point is that the the track analysis agrees with the jets, so there isn't something in the jet finding that's creating the high v2, and that the tracks agree with Shengli, so that Tristan's methodology is validated (as is Shengli's).  But they're looking at very different physics.

The large value of v2 for flow is also interesting in terms of what all this means.

Cheers,

Rosi

On Tue, Aug 29, 2023 at 8:58 AM Ma, Rongrong via Star-hp-l <star-hp-l AT lists.bnl.gov> wrote:

Hello Tristan

Thanks for looking into this quickly. 

Shengli's study shows that more than 50% of the v2 signal observed at 3-4 GeV/c is due to non-flow effect. After non-flow subtraction, the real v2 signal decreases with pT above 3 GeV/c, while your result shows a clear increasing trend. If I follow the trend of Shengli's result, the real v2 could be very small at high pT. So, I think we probably should at least label your plot as "v_2^obs", and state clearly that the non-flow contribution could be very large and has not been subtracted yet. This also raises the question of how much non-flow contamination is there in your Isobar results. 

Best

Rongrong

On Aug 29, 2023, at 12:03 AM, Tristan Protzman <tlp220 AT lehigh.edu> wrote:

Hi Rongrong,

I have managed to throw together a quick v2 on part of the data down to 0.5 GeV, see slide 16 of https://drupal.star.bnl.gov/STAR/system/files/preliminary_figure_request_OO_v2.pdf.  I believe the open blue squares in Shengli's results are the closest method to what I am using, where we see general agreement interpolating between the 20-40% and 40-60% with the non-subtracted EPD based method.  

Cheers,

Tristan 

On Aug 28, 2023, at 12:10 PM, Ma, Rongrong via Star-hp-l <star-hp-l AT lists.bnl.gov> wrote:

Hello Tristan

I would like to follow up on the comment I brought up during the PWG meeting about the single track v2, which if I remember correctly is about 0.15 at 4 GeV/c. On the other hand, slide 9 of https://drupal.star.bnl.gov/STAR/files/OO-ShengliHuang-QM2023_2.pdf shows that charged hadron v2 is about 0.075 at 2.5 GeV/c in 20-40% centrality. Have you looked at single track v2 down to say 2-3 GeV/c, and check if your results are consistent with Shengli's? Or at least we should put both results together and see if we have a consistent picture. 

Thanks. 

Best

Rongrong

On Aug 25, 2023, at 4:33 AM, webmaster--- via Star-hp-l <Star-hp-l AT lists.bnl.gov> wrote:

Dear Star-hp-l AT lists.bnl.gov members,

Tristan Protzman (tlp220 AT lehigh.edu) has submitted a material for a review,  
please have a look:
https://drupal.star.bnl.gov/STAR/node/64873

Deadline: 2023-09-03
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Rosi Reed
Associate Professor, Physics Department
Lehigh University
(610)758-3907
16 Memorial Drive East Office 406
Bethlehem, PA 18015

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Rosi Reed
Associate Professor, Physics Department
Lehigh University
(610)758-3907
16 Memorial Drive East Office 406
Bethlehem, PA 18015

she/her/hers

Click here to sign up for an appointment