STAR Flow, Chirality and Vorticity PWG

[Rongrong Ma <marr AT rcf.rhic.bnl.gov>]

Date: 04/29/2022

 

Participants: Diyu Shen, Aihong Tang, Dhananjaya Thakur, Jinhui Chen, Jiangyong Jia, Barbara Trzeciak, Hanna Zbroszczyk, Mattew Posik, Prithwish Tribedy, Raghav Kunnawalkam Elayavalli, Subhash Singha, Xiaofeng Luo, Yi Yang

 

Title: Observation of the electromagnetic effect via charge-dependent directed flow in Au+Au, Ru+Ru and Zr+Zr collisions at sNN = 200 GeV

PAs: Jinhui Chen, Aditya Prasad Dash, YuGang Ma, Diyu Shen, Subhash Singha, Aihong Tang, Dhananjaya Thakur, Gang Wang

Target journal: PRX

Proposal page: https://drupal.star.bnl.gov/STAR/blog/dshen/observation-charge-dependent-flow-and-its-implication-electron-magnetic-field-auau-200gev

Presentation: https://drupal.star.bnl.gov/STAR/system/files/PWGC_0.pdf

 

The PWGC panel previewed a paper proposal from FCV PWG. The panel found that the analysis is mature and should move forward. In terms of the target journal, most of the conveners, unfortunately, do not have much experience with it since PRX is a relatively new journal. So we suggest to leave it to PAs, PWG and GPC to decide. 

(C - Comment, Q - Question, A - Answer)
Fig. 2
C: the sign of the v1 slope difference between positive and negative particles illustrated in this figure is opposite to the analysis. Would be good to make it consistent. 
A: will do. 

Fig. 3
Q: how serious should one treat the trends of these curves? 
A: these curves are mainly meant for illustration of the sign of v1 slope difference for transported quarks and EM-field effect. The curve for transported quark is based on UrQMD calculation, while the curve for EM-field is also based on theoretical model. 
Q: if the transport quark effect is based on UrQMD, why is its dependence on centrality (decreasing) different from what's shown in Fig. 7 (increasing)?
A: this curve is indeed based on UrQMD but for 11 GeV, while Fig. 7 is for 200 GeV. Will make it consistent to avoid confusions.

Fig. 7
Q: why is IEBE-VISHNU + EM-field calculation negative in all centrality bins?
A: this is because this calculation only considers EM-field effect, no transported quarks.
Q: is there a model which incorporates both transport quarks and EM-field effect so it can be compared to data?
A: we are not aware of such calculation at the moment
Q: can we say anything about EM-field effect in semi-central collisions?
A: that is difficult because we do not know how large the transport quark effect is. We do not have any theoretical guidance. 
Q: is there anything additional we can learn from the Isobar data? how should one compare the Au+Au and Isobar results?
A: the Isobar data confirm the negative v1 slope for proton and anti-proton seen in peripheral events. It does not add much physics, but could be useful for constraining theoretical calculations. Comparing Au+Au and Isobar results is difficult as many other aspects (system size, conductivity of the medium, transport quark contribution) are different in addition to the magnetic field difference. 
Q: can one compare Au+Au and Isobar at similar Npart values to take out system size difference?
A: we can, but still the medium conductivity and transport quark contribution could be different. 

Conclusion
Q: how do we understand the behavior for pions and kaons, what are different from protons?
A: pions are dominated by resonance decays, so one does not expect EM-field induced v1 slope splitting. Kaons contain strange quarks, and there are other v1 measurements of strange quarks indicating positive v1 splitting. We will work on expanding on these details, but this does not affect the physics message of this paper, which is mainly based on protons. 
Q: can we really say that the negative values in peripheral events can only be explained by EM field?
A: we can tune down this statement.

C: if one can somehow compare the results from Au+Au and Isobar which have different EM field, this could potentially strength the paper.