sPHENIX bulk physics topical group

[Jamie Nagle <jamie.nagle AT colorado.edu>]

Hello Milan, Joern, Yuko, Rachid (cc sphenix-bulk list),

Dennis asked if I would take a look at the latest results from your very interesting (and challenging) pp correlations analysis.

https://indico.bnl.gov/event/26572/contributions/102833/attachments/59885/102885/ppFlow_20250211.pdf

I think you have identified two of the key issues and wanted to follow up on further quantification.

1.    Event selection and double interactions.    Your initial studies look promising.     For the later 0.5, 0.05, 0.005% selections, can you show those on the EM Tower multiplicity distribution?    Is there an upper end cut, to remove very, very high end background?     For each of these percentage selections, can you quote the estimated purity (e.g., 95%, 98%), and how that is determined.    From the correlation plots, I suspect that some form of background / streak events are sneaking in for the 0.005% selection -- where you see a dramatic narrowing of the distribution.   

I am also curious if you considered using the sEPD for event selection, and EM Towers for the correlation as a comparison.

2.    With a selection of |delta-eta| > 1.2, that is rather modest, and there are likely to be non-flow near-side jet contributions.   A near side contribution is rather problematic and one could consider an even wider cut since you have statistics.    For the away-side non-flow, a group of us implemented a number of non-flow subtraction techniques in one package:

https://journals.aps.org/prc/abstract/10.1103/PhysRevC.100.024908

Sanghoon and Blair are two sPHENIX members who have the code package, and I am sure that you can request that from them.

Then you can take the correlations on slides 9 and apply a decomposition -- where you can have a low multiplicity (LM) and high multiplicity (HM) selection.    I would be glad to help, though those two are the real experts.

3.   A third one relates to what you are really measuring.      Imagine you see no clear signal.    How does one compare that to a STAR result (note they had a special low luminosity pp running in 2024 specifically for this purpose -- presumably using their EPD)?   How does it compare to a theory prediction?     Thus, I think it is important to take some Monte Carlo with known flow and see what you would measure from the EMCal Towers -- not clear with your cuts that you are mostly measuring photons from pizero and eta decay.    You could even look at a AuAu MC sample and ask how many towers (in peripheral events) that pass your cuts are photons, and what else...   How does the v2 extracted by this method compare to some truth value -- for example pizero/eta --> photons truth v2 between 0.6-3.0 GeV...

I really appreciate your effort on this front, and am happy to clarify / help resolve any of the points above.

Sincerely,

Jamie

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|| James L. Nagle   
|| Professor of Physics, University of Colorado Boulder

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