Charmonium Transport in Heavy-Ion Collisions at the LHC

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This review deals with a detailed discussion on the properties of charmonium dynamics in hadronic collisions at ultra-relativistic incident energies. The manuscript is written in a transparent and clear way and deserves publication. 

There are only some minor issues that the authors may include in their revision: 

1) Introduction: here it would be appropriate to include the transport models, for instance, HSD and UrQMD, by giving the appropriate references.

In my opinion, it would be better for a general reader (not so familiar with this task) to give a slightly more general review on other models than the used one. 

In this respect, I would suggest the authors to remove some of the self-citations by replacing them with the other transport models (HSD, UrQMD). 

2) In many places of the manuscript the pp-experimental cross sections are indicated. My question here is: what about pn- and nn-binary cross sections? how these are handled within the model? A short indication on this remark might be helpful. 

3) As i have understood, the model applied here deals with a global temperature of the  source under consideration (fireball for example). However, i rather believe to local equilibrium than to a global one.  

The authors give a comment on this issue at page 14, line 408 and below. Is there any attempts to extend the present approach to local equilibration? Can be the present model applied to situations with a radial temperature profile? Some short comment on this topic might be fine, but not necessary. 

Author Response

• 1) We expanded the discussion including the SHM and included references to HSD and UrQMD calculations of charmonia when discussing SPS results (we could not find LHC applications that include regeneration), and we were able to reduce the overall amount of self-citations by ~35% (from 32.7% to 20.3%), although some of those cuts may imply lack of information.
• 2) We included a remark in Sec. 2.4 on treating pn and nn collisions as identical to pp for the partonic production processes considered here.
• 3) The remark on page 14 is more geared toward the assumption of thermalized charm quarks, rather than a nontrivial spatial profile in the fireball. One obstacle in implementing the latter is that the local equilibrium limit is much more difficult to define, although work in that direction is in progress. We added a remark on that in the second half of Sec. 2.4 where the fireball model is described.

Reviewer 2 Report

Comments and Suggestions for Authors

Comments for author File: Comments.pdf

Author Response

We thank the referee for the detailed remarks. With few exceptions (L15, L29-32) we implemented all of his/her comments.

As for the shadowing (L197), we prefer to use our parameterization of the data as constructed in the ms., but it is not inconsistent with available shadowing models, e.g., the EPS09 parameterization shown in ref. [43] or the EPPS16 and nCTEQ15 parameterization (see Ref. [46]), as well as the midrapidity data from [47] (for which we assume the same parameterization). We added pertinent remarks and these 3 references in the discussion of Fig. 4 in the first paragraph on page 7.

Reviewer 3 Report

Comments and Suggestions for Authors

In the paper entitled “Charmonium transport in ultra-relativistic heavy-ion collisions at the LHC”, the authors have studied the charmonium production in heavy-ion collisions at the LHC by using a previously constructed semi-classical transport approach. This paper is interesting and useful for the community of high energy collisions. 

Some comments

1) In the title, “ultra-relativistic” and “LHC” have repetitive meanings. Maybe, you may remove “ultra-relativistic” or “at the LHC”.

2) The pseudo-critical QCD transition temperature is about 160 MeV, however, the dissociation temperature used in this paper is much high (240-360 MeV). May you explain the difference?

3) In Figures 5, 11, 12, and 14, how to judge the quality of fitting? Maybe, you may provide chi^2/ndof.

4) At the mid-rapidity and forward rapidity, what is the difference in temperature? Which one is larger?

5) For J/psi and psi(2S), what is the difference in their source temperatures? Which one is larger?

6) In central, semi-central, and peripheral collisions, what is the difference in temperature? Which one is larger? Why?

7) In pp and Pb-Pb collisions, what is the difference in temperature? Which one is larger? Why?

8) You have pointed out that psi(2S) has a much smaller dissociation temperature than J/psi. Does this mean the multi-scenario of chemical freeze-out?

 

Author Response

We thank the referee for his/her useful suggestions, which we have implemented as follows

• 1) We have amended the title accordingly, reads much better now!
• 2) This is of course a long-standing question in the field, i.e., whether some hadronic states can survive in the strongly coupled QGP (or even are at the origin of the strong-coupling properties). There is, in fact, increasing evidence for that, and our calculations are based on a systematic quantum-many body approach that supports that (see, e.g. Liu+Rapp, PRC97(2018) = [33] or the review in Ref. [35]. Also note that the commonly quoted “pseudo-critical temperature”, Tpc~160MeV, is the one related to the chiral transition (the chiral condensate has dropped down to ~50% of its value), while the one related to de/confinement, the Polyakov loop, shows a very gradual change for temperatures well above that.
  
  The different binding energies of quarkonia (E_B~1 GeV for Y(1S) down to very small values for psi(2S) or excited Y states) render them excellent probes of this question.
  
  We added a brief remark on that toward the end of the 1^st paragraph of the introduction.

• 3) We did not really carry out a fit as the input values for the charmonium transport properties have been fixed in previous work; however, this will likely be done in the future. We refer the referee to Du et al., PLB796(2019)20 where a statistical data analysis has been carried out for Y data (as Y states are less prone to regeneration which reduces theoretical model uncertainties).
• 4) At midrapidity the charged-particle multiplicity is about 20% larger than at forward y ~2.5-4. This implies that the initial T, with everything else equal, is about (1.2)^1/3 ~ 6% larger at midrapidity compared to forward y. We added a pertinent remark in the context of Fig. 6.
• 5) In the transport approach, both J/psi and psi’ are first suppressed and, once the temperature is below the dissociation temperature, regenerated continuously, as governed by their reaction rates, see Fig. 7 (the pertinent dissociation temperatures, quoted at the end of Sec. 2.4, are taken from in-medium T-matrix calculations and reflect the hierarchy in their binding). As for the pT spectra for regeneration, we approximate them with an average source temperature, roughly reflecting an average over their production time, i.e. 180 MeV for the J/psi and 160 MeV for psi’, which are quoted in the paragraph below eq. (19).
• 6) The “source temperatures” for the regenerated charmonia are assumed to be the same across all centralities, we added this information in the paragraph after eq. (19).
• 7) In pp collisions, it is assumed that no medium is formed and the pertinent spectra (both for the initial conditions in AA and for the denominator of the RAA) are simply parameterized from pp data.
• 8) Yes, absolutely, this is why we emphasize the concept of sequential regeneration, which is also directly related to point 2 above. In particular, we believe that the deviation of the psi’/Jpsi ratio from the equilibrium value provides a direct signature of this, as emphasized again in the conclusions.