STAR HardProbes PWG

[Barbara Trzeciak <barbara.trzeciak AT gmail.com>]

Date: 05/31/2024

Participants: Ziyang Li, Shuai Yang, Rongrong Ma, Prithwish Tribedy, Wangmei Zha, Kaifeng Shen, Hanna Zbroszczyk, Isaac Mooney, Jae Nam, Kaifeng Shen, Nihar Sahoo, Nu Xu, ShinIchi Esumi, Xiaoxuan Chu, Yi Yang, Zebo Tang, Sooraj Radhakrishnan, Barbara Trzeciak

Title: Very low pT J/ψ production in Au+Au collisions at √sNN =200 GeV through the dimuon channel at STAR

PAs: Ziyang Li, Zhen Liu, Rongrong Ma, Kaifeng Shen, Zebo Tang, Wangmei Zha

Target journal: Chinese Physics C

Proposal page: https://drupal.star.bnl.gov/STAR/blog/zli1/Very-low-pT-J%CF%88-production-AuAu-collisions-200-GeV-through-dimuon-channel-STAR

Presentation: https://drupal.star.bnl.gov/STAR/blog/zli1/PWGC-Preview-very-low-pT-Jpsi

The PWGC panel previewed a paper proposal from HP PWG. The panel found that the analysis is mature and results are important and interesting, and the paper should move forward. The journal choice was also found to be appropriate. The following points were discussed.

Q: Figure 5: The difference between e+e- and mu-mu+ results seems to be due the rapidity coverage, is this correct?

A:  Yes, based on the model. There might also be some statistical fluctuations.

Q: Would it help to combine e+e- published results reducing y coverage to 0.5 and the \mu+\mu- results ? 

A:  Probably wouldn’t, it would reduce the e+e- statistics in ~half.

Q: What is N+N and N+S ? The effect is mostly in peripheral collisions.

A:  It’s a case when a photon interacts with the whole nucleus or only with spectators.

Q: Figure 3: legend for the curve is missing

A:  We will add.

Q: Figure 5: Yield in data seems not to change vs N_part.

A:  Yes, data is more consistent with the N+N scenario.

Q: Figure 5: Is this only for coherent contribution ? What happens for incoherent vs N_part ?

A:  pT cut is < 0.1 GeV/c, therefore the incoherent production contributions should be very small.

Q: Figure 5: can we do this for phi meson?

A:  It might be possible with iTPC by lower pT cut, or with lower magnetic field data.

Q: s8: why do you need polarizaton here ?

A:  It’s because the acceptance is polarization dependent, but the effects is very small.

Q: Figure 4: You show R_AA up to 10 GeV/c, is this from this measurement or other published ?

A:  It goes up to 7 GeV/c, and yes it’s all new analysis. They are consistent with 2014 published results.

Q: s8: Why do we see so large charge dependence of the efficiency ?

A:  Because these are efficiencies in different phi bins.

Q: Figure 6: conclusion on the incoherent component is a bit tricky, the errors in data are large and subtraction is difficult. You need to use a different fit function to see if the tail is sensitive to it.

A:  We will check this effect. 

Q: Figure 6: do you take into account momentum resolution when comparing to data, is there GEANT simulation?

A:  It’s just a model calculation. But the momentum resolution should be corrected for in the efficiency correction. 

Q: Figure 6: What is the bigger physics picture ?

A:  We see a drop and peak at very low pT, it's an important test of the interference effect. The slope at higher pT is sensitive to the size of the nucleus. 

Q: But you don’t mention this in your conclusion.

Q: In order to have interference you need symmetry between the left and right going nucleus, but you have here spectator on one side

A:  E.g. for Cu+Au we still see interference, just aptitudes are different. 

Q: Figure 5: suggest to reverse order of Fig. 5 and Fig. 6.

A:  Enhancement is already shown on Fig. 3. We show Fig. 5 first because we compare here N+S and N+N scenario, and then on figure 6 we only show N+S scenario as it describes data better. 

Q: Figure 5: we should explain the difference between the two results. 

A:  We will keep checking, but the difference between the two is less than 2sigma when mu+mu- results are scaled down to take into account the different y coverage.

Q: Figure 5: Why in EPA muon yield is larger ? 

A:  That’s because of the rapidity distribution shape.