String Interactions as a Source of Collective Behaviour
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThis topical review is an interesting one for a broad reader and should be published in my opinion. However, i would like to make several suggestions towards its improvement.
First, it is not a first review on the collectivity in small systems combined with the similar effects in heavy-ion collisions. There are several previous papers that deserves to be cited. For example:
S. Schlichting, P. Tribedy, Advances in High Energy Physics, Volume 2016, Article ID 8460349;
Wei Li, Mod. Phys. Lett. Volume 27, Article ID 1230018.
The above, but not only, refs would help a reader to better determine the place and role of the given topical review.
The idea to apply Lund string model for an explanation of collectivity effects in both small and large systems is interesting and deserves to be reviewed regarding the particular results of its application. It provides an alternative to the explanations based on the extension of QGP formation mechanism to small systems. This alternative explanation is based on the string hadronisation dynamics amended with strings' shoving and ropes formation.
Regarding string formation themselves, it would be desirable to explain first the dynamics of string formation and to describe why the string formation should be taken as a dominant mechanism. What are the arguments? I mean here remarks made by D. Diakonov that " it is sufficient to exceed the mass of one additional pion to make the flux tube energetically unfavourable" (D. Diakonov, arXiv: hep-ph/0406043v1.) The statement concerns the quark-antiquark interaction energy.
Second, the partons are colored objects and color reconnection is an important issue for the color topologies. But, what is then their scattering cross-section (page 2, line 47)? This problem related to confinement should be mentioned, at least.
Next, the string is a one--dimensional object (or a tube?), what is definition for the "radius" of this object (Caption to Fig. 3)? What is definition for the multiplicity function w (page 2, line 60)? How multiplicity is related to the sum of transverse energies (page 3, line 66) and what is the role of elastic scattering (with multiplicity 2) in this relation? The above items seems deserve a discussion especially in a review type of article.
Author Response
I thank the referee for their comments on my manuscript. Their comments are addressed below.
1) "First, it is not a first review on the collectivity in small systems combined with the similar effects in heavy-ion collisions..."
The purpose of this review is not to serve as a general review of collectivity in small systems, but rather as a specialized review concerning string interaction effects. I do, however, agree with the referee's comment that more references, placing the review in its context, could be good. I have thus added more explanatory text to this effect in footnote 1, as well as cited the two suggested reviews along with others.
2) "I mean here remarks made by D. Diakonov that " it is sufficient to exceed the mass of one additional pion to make the flux tube energetically unfavourable" (D. Diakonov, arXiv: hep-ph/0406043v1.)..."
To my best understanding, the arguments by Diakonov pertains to the string flux tube as an explanation for partons binding together in hadrons (baryons, specifically), and *not* string fragmentation as such. In fact, the string model agrees with the statements made by Diakonov in the sense, that also here, the string will break, producing hadrons. In fact, Fig. 6 in the paper could have been taken from a paper on baryon production in the string model, through the junction mechanism.
3) "Second, the partons are colored objects and color reconnection is an important issue for the color topologies. But, what is then their scattering cross-section (page 2, line 47)? This problem related to confinement should be mentioned, at least."
A brief discussion of the 2->2 perturbative cross section and its relation to the hadron-hadron cross section has been included.
4) "Next, the string is a one--dimensional object (or a tube?), what is definition for the "radius" of this object (Caption to Fig. 3)?" this is exactly the assumption (1+1D) which is relaxed to allow strings to interact, as explained in the beginning of sec 3.2. This point has been emphasized.
5) "What is definition for the multiplicity function w (page 2, line 60)?" The exact definition of w is unimportant, as it is not used in the Angantyr model, as explained in the following line. I have removed the reference to a function name, and replaced with the wording "an ad hoc function".
6) "How multiplicity is related to the sum of transverse energies (page 3, line 66)" the point here is not relating multiplicity to transverse energy, but reproduction of centrality measures. This has been clarified in the text.
7) "And what is the role of elastic scattering (with multiplicity 2) in this relation?" Purely elastic scattering (p on A or A on A) plays little to no role in this relation, and the point is therefore not further discussed.
Reviewer 2 Report
Comments and Suggestions for AuthorsThe author reviews the microscopic string fragmentation model puting forward the idea of a possible description of collectivity effects without resorting to Quark Gluon Plasma as a source. The analysis focuses on strange production and flow but mentions other predictions obtained with the hypothesis of QGP formation as jet quenching, altered resonance production, etc.
The topic is of great importance in the area of ultrarelaltivistc heavy ion collisions and QCD studies where the formation of QGP is supposed to explain a number of phenomena. The critical comments and extensive bibliographic review represent a contribution to the research area.
The papers is presented as a minireview but does not consider experimental data beyond LHC experiments, - in particular does not refere to previous RHIC results -.
The conclusions go in line with what is discussed throughout the text. It provides a good set of references on the ongoing efforts to construct models free of the widely accepted QGP hadronization scheme.
It is a valuable brief review with critical ideas and a broad view of the research field.
Author Response
I thank the referee for their comment regarding the lack of reference to RHIC results. I have emphasized this lack in footnote 1, and provided a reference.
Reviewer 3 Report
Comments and Suggestions for Authorsplease see the attached file
Comments for author File: 
Comments.pdf
Author Response
I thank the referee for the very detailed and informative letter.
Overall I agree with most of the comments. It is indeed not obvious why a starting point in perturbative QCD can give a correct description of the soft physics governing heavy ion collisions. However, I find it difficult to address all points of the referee report point by point, without potentially turning this review into something it is not. What it is, is a mini-review of the string interaction models as implemented in Pythia. I have, however, attempted to include what I find to be the most pertinent points in the manuscript, as listed below:
1) I have extended the section describing the perturbative QCD starting point of the MPI model. This to make it clear for the reader that a particular model choice is made. I have emphasized, at the end of this section, that this is not the only possible choice, referring to CGC and the EPOS model as examples.
2) I have highlighted the possible interpretation of of the regularization procedure in the Pythia MPI model as saturation physics "through the back-door".
3) I have highlighted that that there is some similarity between the cut Pomeron approach and Angantyr model, when it comes to secondary absorptive collisions, and stated that this is currently only superficially explored in the literature.
4) I have pointed out that string interactions can lead to QGP formation (Bautista:2019mts,Ramirez:2020vne), ie the opposite conclusion than from the Pythia string interaction models.
