Direct Photon Production in Heavy-Ion Collisions: Theory and Experiment
Round 1
Reviewer 1 Report
In this paper, the authors have reviewed experimental results on direct photon production in pp, pA and AA collisions at SPS, RHIC and LHC and discussed comparisons with theoretical predictions. In addition, the authors have presented some predictions of direct photon spectra and collective flow at NICA and FAIR. This paper is useful for the community and worth publishing.
Some minor revisions are needed before publication.
L1: provide possibility to test à provide possibility a chance to test
L10: SPS, RHIC and LHC à the Super Proton Synchroton (SPS), the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC)
L13: NICA and FAIR à the Nuclotron-based Ion Collider fAcility (NICA) and the Facility for Antiproton and Ion Research (FAIR)
L26: low-$p_T$ à low transverse momentum (low-$p_T$)
L31: pp and p-A à proton-proton (pp) and proton-nucleus (p-A)
L32: perturbative QCD à perturbative quantum chromodynamics (pQCD)
L33: A-A à nucleus-nucleus (AA)
L35: NICA and FAIR à the Nuclotron-based Ion Collider fAcility (NICA) and the Facility for Antiproton and Ion Research (FAIR)
L46: $\sqrt{\Delta\phi^2+\Delta\eta^2} ~ 0.4.$ à $\sqrt{\Delta\phi^2+\Delta\eta^2} ~ 0.4,$ where $\phi$ and $\eta$ denote the azimuthal angle and pseudorapidity of photon respectively.
L51: $\sqrt{s}=200$ à the center-of-mass energy $\sqrt{s}=200$
L69-70: In the caption of Fig.1, please add the reference.
L77: TEVATRON and HERA à Tevatron proton-antiproton collider (Tevatron) and Hadron-Electron Ring Accelerator (HERA)
L81: NLO à next-to-leading order (NLO)
L82: NNLO à next-to-next-to-leading order (NNLO)
L90: $N_{\rm coll}$ à $N_{\rm coll}$ is the number of binary collisions and
L96: $\sqrt{s_{\rm NN}}=200$ à center-of-mass energy per nucleon pair $\sqrt{s_{\rm NN}}=200$
L109-110: Paquet et al., v. Hees et al., Chatterjee et al. should be replaced by the numbers of references. The information on titles, journals, etc. should be included in the list of references.
L110: transport model PHSD à transport model PHSD (Parton-Hadron-String Dynamics)
L138: (syst). à (syst), where $N_{\gamma,dir}$ and $N_{ch}$ denote the numbers of direct photons and charged particles respectively, and $y$ and $\eta$ denote the rapidity of direct photon and the pseudorapidity of charged particle respectively.
L142: minimal $p_T$ à minimal $p_T$ ($p_T^{min}$)
L143: hadron gas à hadron gas (HG)
L206-208: The full names of CT, NNPDF, ABMP, MMHT, nCTEQ, EPPS, nNNPDF, and TUJU are needed, if available.
L234: UrQMD à Ultra-relativistic Quamtum Molecular Dynamics (UrQMD)
Author Response
Thank you for careful reading. All suggestions were implemented except
(R) L35: NICA and FAIR à the Nuclotron-based Ion Collider fAcility (NICA) and the Facility for Antiproton and Ion Research (FAIR)
(A) NICA/FAIR already introduced earlier
(R) L206-208: The full names of CT, NNPDF, ABMP, MMHT, nCTEQ, EPPS, nNNPDF, and TUJU are needed, if available.
(A) We consider these abbreviations as sufficient labels for our purposes. Discussion of specific features of different parameterizations would be interesting but not suitable for this paper.
Best regards, Authors
Reviewer 2 Report
This is well written except for a few typos and jargon. Introduction can be improved for a general reader. Please explain all symbols used in equations and define acronyms before using them.
Line 3: Replace "staged" with "stages"
Line 6: It is not clear how is the scaling realted to these hot or cold hadronic matter properties?
Line 18: How to distinguish direct photons from decay photons?
Line 33: Replace "collisions and an" with "collisions and the"
Line 33: Replace "and experimental" with "with the experimental"
Line 43: Missing article 'a'- "a complicated" instead of "complicated"
Line 51: Missing article 'the'- "the theoretical" instead of "theoretical"
Line 52: Replace "the contribution" with "contribution"
Fig. 1: y axis label different from the equation in line 59.
Line 67: Explain "higher-twist processes" briefly.
Line 76: Explain "Jetphox package" briefly.
Line 110: What does PHSD mean?
Line 121: Replace "consist in" with "consist of"
Line 153: Replace "increase" with "an increase"
Line 164: Replace "is originated" with "originates"
Line 169: Replace "can not" with "can"
Line 170: Replace "within" with "with" and also "nor" with "nor with"
Line 171: Replace "significant" with "a significant"
Line 172: Replace "developed" with "a developing"
Line 185: Replace "extending" with "extension"
Line 187: Replace "be the system" with "the system be"
Line 190-191: Replace "the unity" with "unity" and "considered as a hint to" with "a hint of".
Line 197: Replace "the unity" with "unity"
Line 213: Replace "measurement" with "measurements"
Line 214: Replace "performed" with "performed in"
Line 240: Replace "realistic" with "a realistic"
Line 261: Replace "provides an" with "provides"
Line 264: Replace "firm" with "a firm"
Line 266: Replace "very" with "well"
Author Response
Dear Referee, thank you for detailed review. We implemented most of your suggestions except the following:
(R) Line 6: It is not clear how is the scaling related to these hot or cold hadronic matter properties?
(A) Scaling is not related to hot matter properties, it appears in the first part of the sentence, while second part "get insight into the hot and cold hadronic matter properties with soft photons" concerns thermal emission. We think, the sentence is sufficiently clear as is.
(R) Line 18: How to distinguish direct photons from decay photons?
(A) Here we introduce definition of direct photons. The methods of their extraction are discussed later. We can not move this discussion to the very beginning of introduction.
(R): Fig. 1: y axis label different from the equation in line 59.
(A): We do not have possibility to correct axis in Fig.1 as it is a property of the ALICE collaboration
(R) Line 76: Explain "Jetphox package" briefly.
(A) Not clear how one can explain this package. We mentioned what is implemented in it and provided reference.
Reviewer 3 Report
In the first parts of the manuscript, the authors give a concise but comprehensive review of direct-photon measurements in heavy-ion collisions.
The main features of momentum distributions of photons produced at RHIC and LHC energies are outlined and confronted with theoretical calculations.
Thus, the manuscript gives the reader a quick introduction to the problem of photon production in heavy-ion collisions.
The second part of the manuscript is devoted to the analysis of the photon production at NICA and FAIR energies. This part could be instrumental in the preparation of future experiments.
I deem the manuscript very useful and recommend it for publication.
However, I would like to suggest some minor modifications to the manuscript:
1.-line 74. The authors prize the work by Gordon and Vogelsang, whereas sometime earlier the independent calculations of the direct photon production were published by A.P. Contogouris, B. Kamal, Z. Mcrebashvili, and F.V. Tkachov, in Physics Letters B 304 (1993) 329-333 where a more advanced calculation technique was used. I suggest citing this paper also.
2.-lines 131 -148 In this discussion
a) I would quote explicitly the value of \alpha obtained in the old fit (\alpha=1.23?) see line 138.
b) In Ref. [Adare Phys. Rev. Lett 123, 022301 (2019)] a higher value \alpha =1.25 was obtained, see Fig. 2. I think this value should also be included in the discussion. Why it is larger than the values \alpah=1.1 following from Fig. 6 of the present manuscript?
c) Line 147, I would suggest specifying clearly where the value \alpha=1.6 can be found in Ref. [25]. As I see it follows from Fig. 6b there.
d) The obtained result \alpha=1.1 requires some additional discussion. Does it mean that actual photon production processes are less effective than hadron gas, QGP, and pQCD? What could they be after all?
3. lines 167-173. If it is possible, it would be good to illustrate the discussed behavior of the elliptic photon flow in collisions at 200GeV, to confront this figure with Fig.7.
4. Figure 8. Here the reference for theoretical lines is not given. If this is the work [Shen et al, Nuclear and Particle Physics Proceedings 289–290 (2017) 161–164, https://doi.org/10.1016/j.nuclphysbps.2017.05.034] I would also mention how the ratio was recalculated from p+Pb to p+Au collisions.
5. lines 198-199. This sentence sounds as if the absence of thermal emission in Fig. 9 for collisions at 5TeV shed some doubts on the data shown in Fig. 11. However, the incoming proton momenta are dramatically different in these two collisions. The energy deposit from the passing proton to the surrounding medium could be essentially different (stronger transparency at the higher momentum).
6. Comments about figures
a) Fig. 8 seems to be blurred compared to others.
b) Figs. 13,14,15 the inscriptions on the axes are too small so that they are hardly readable.
Author Response
Dear Referee, thank you for very valuable suggestions. We implemented most of them, concerning the rest we have the following comments
2.-lines 131 -148 In this discussion
b) In Ref. [Adare Phys. Rev. Lett 123, 022301 (2019)] a higher value \alpha =1.25 was obtained, see Fig. 2. I think this value should also be included in the discussion. Why it is larger than the values \alpah=1.1 following from Fig. 6 of the present manuscript?
> Value 1.25 was not really extracted in this paper, but rather fixed from some theoretical considerations and then tested if it agrees with data. Once this parameter was released in the fit, a smaller value ~1.23 was obtained. In general, PHENIX published several values of \alpha depending on paper (amount of data used in scaling) and pT range. Uncertainties of extracted values are so large that all numbers are consistent. We prefer to discuss only the very latest results with the largest data sample.
5. lines 198-199. This sentence sounds as if the absence of thermal emission in Fig. 9 for collisions at 5TeV shed some doubts on the data shown in Fig. 11. However, the incoming proton momenta are dramatically different in these two collisions. The energy deposit from the passing proton to the surrounding medium could be essentially different (stronger transparency at the higher momentum).
> We do have some doubts in the PHENIX results: one could expect larger fireball at LHC energy and stronger thermal contribution at LHC. Prompt photon kinematics also suggests stronger gluon-quark contribution (due to rising gluon PDF) at LHC energies. However, as uncertainties are large and all data are consistent with each other, with no thermal photon yield and with hydro calculations, one can not make firm conclusion. So we prefer to leave sentence as is.
Reviewer 4 Report
The authors of this paper give a brief review on experimental results of direct photon production in relativistic heavy ion collisions. Some important results of major experiments (PHENIX, STAR, ALICE, ATLAS, etc.) in various collision systems (p+p, p+A, and A+A) are overviewed. The paper is publishable in the present form provided minor corrections are made.
1) Line 109: references for hydrodynamic descriptions are needed, instead of listing some authors.
2) Line 137: The dashed line is the fit to the PHENIX data that has been published previously, not fit to preliminary data.
3) The resolution for Fig.8 and Fig.9 is not good enough and should be improved.
4) Fig.10 shows the sensitivity of PDF on direct photon production at low colliding energies, what are sensitivities at high energies, e.g, 200GeV and 7TeV?
5) It would be better if the authors make some discussions on the connection
between real and virtual photon production.
Author Response
Review 4
Dear Referee, we implemented your suggestions except the following:
3) The resolution for Fig.8 and Fig.9 is not good enough and should be improved.
> Unfortunately, these are the best quality plots which we managed to get from PHENIX collaboration.
4) Fig.10 shows the sensitivity of PDF on direct photon production at low colliding energies, what are sensitivities at high energies, e.g, 200GeV and 7TeV?
> As at RHIC and LHC one tests PDF in the middle-x region where it is well constrained, the difference between parameterizations is much smaller, see fig.2 where isolated photon yield is calculated with several PDFs.
5) It would be better if the authors make some discussions on the connection between real and virtual photon production.
> We mentioned relation between very-low-mass virtual photons and real ones (Kroll-Wada formula), but dilepton physics is too big subject, it can not be properly discussed in this paper and deserves separate review.
