STAR Flow, Chirality and Vorticity PWG

["Adams, Joseph R." <adams.1940 AT osu.edu>]

Hi all,

Please refer to my FCV presentation from 2021 regarding today's discussion on momentum-conservation effects which can affect strongly measurements of event-plane resolution at low energy/fixed target, where multiplicity is particularly low: https://drupal.star.bnl.gov/STAR/system/files/2021-03-31_PolarizationAt3GeV_v2.pdf

Keep in mind that we use the three-subevent method for measuring resolution (https://arxiv.org/abs/nucl-ex/9805001) in fixed-target mode, since the collision system is not symmetric. We have the "main" subevent which is used to measure the Psi_1 that will be used in the measurement of the observable, and two "reference" subevents from which we measure Psi_1 and compare to the Psi_1 from the main subevent in order to measure the resolution of the main subevent. This resolution on the main subevent is used as a correction on the observable which makes correlations with Psi_1 from the main subevent.

Importantly, the resolution of the main subevent should definitely NOT depend on your choice of subevents; however, it can be artificially modified by non-flow effects. In particular, momentum conservation will artificially enhance the resolution.

In my slides from the link above, I was initially searching for systematic mistakes (systematic effects that weren't accounted for) and found that the polarization observable depended on EPD ring (slide 12), which seemed wrong. I have encouraged those performing fixed-target analyses to also check the dependence of their observable on EPD ring. Realizing other strange behavior in the smaller rings, and also that those rings were substantially farther-forward in rapidity than the beam rapidity, I decided to use only the outer EPD rings (13-16, inclusive). This restriction in eta helps remove momentum-conservation contributions to the subevent's resolution. Importantly, there is no observable dependence of polarization on subevent size within rings 13-16 (i.e. when using only ring 13, rings 13&14, rings 13-15, and rings 13-16). 

Now, being satisfied with the main subevent, one must choose reference subevents. Again, any dependence of the main subevent's resolution on reference subevent choice suggests problems. On slide 16, you can see the resolution as a function of reference subevents of eta width 0.1 in the TPC. In the lower left, we are choosing subevents that are close in eta to the EPD, so unsurprisingly, we see artificial enhancement. Instead, we want to see the resolution reach a stable point, which we see farther from the EPD. At that point, the choice of reference subevents is arbitrary. For those reference subevents, we reduced their size and saw no significant change in resolution, meaning we are sufficiently free of momentum-conservation effects and other non-flow effects.

At this point, we have selected a main subevent and two reference subevents that show no observable non-flow effects and therefore offer an accurate representation of Psi_1 and its resolution. Again, the resolution is a correction; the uncertainty on that correction (which is simply the statistical uncertainties) is that correction's contribution to the total systemic uncertainty. Read https://arxiv.org/abs/hep-ex/0207026 for more details on systematic uncertainties.

Please let me know if you have any questions, and feel free to offer corrections or clarifications.

Best,

Joey

---

From: Star-fcv-l <star-fcv-l-bounces AT lists.bnl.gov> on behalf of subhash via Star-fcv-l <star-fcv-l AT lists.bnl.gov>
Sent: Monday, January 16, 2023 7:08 AM
To: STAR Flow, Chirality and Vorticity PWG <star-fcv-l AT lists.bnl.gov>
Subject: [Star-fcv-l] FCV PWG meeting on 18/January/2023 Wed. 9:30 AM EDT

 

Dear All,

We shall have our weekly FCV PWG meeting this Wednesday
(18/January/2023) at 9:30 AM EDT. If you would like to present please
let us know. Please try posting your slides by Tuesday. The agenda will
be collected at: https://urldefense.com/v3/__https://drupal.star.bnl.gov/STAR/blog/jjiastar/bulkcorr__;!!KGKeukY!y6Kc06ZAp8I7TBzKNFTSiUHvE2zhEidLU76XXRMZkP6mSFQw5fQoszPOXZF7kwDlxVlY5lwRgIYw4N5ReJByhb_VbS31Vg$

Zoom details are copied below.

Thanks and regards,
Prithwish, Zhenyu and Subhash


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