Heavy-Ion Physics at the CERN SPS H2: NA35, NA49 and NA61/SHINE (With Personal Recollections)

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The author does an historical analysis of the experiments Na35, Na49 and Na61/Shine  done in the SPS at CERN, concerning heavy ions collisions. He shows the highligths of the findings done specially the first signal of quark gluon plasma and the onser of deconfinement .The paper is interesting and remember some important issue of the field that now very often is not realized. The paper deserves publication

Author Response

Comment: The English could be improved to make the research clearer.

Response: The language and the quality of the plots were improved.

Reviewer 2 Report

Comments and Suggestions for Authors

Report on the review article "Heavy-ion physics at the CERN SPS H2: NA35, NA49 and NA61/SHINE" by M. Gazdzicki. 

This article reviews from a personal perspective the heavy-ion physics program carried out at the CERN SPS H2 beam-line by the experiments NA35, NA49, and NA61/SHINE. The aim of these experiments was to study strongly interacting matter under extreme conditions and to explore the phase diagram of quantum chromo-dynamics (QCD). NA35 provided early measurements of particle production and energy density in nucleus–nucleus collisions, giving first indications of new collective phenomena happening at SPS energies. Building on this, NA49 performed high-precision, large-acceptance measurements of strange hadron production, leading to results that gave evidence for the onset of deconfinement at low SPS energies. NA61/SHINE continues this program with an extended and systematic scan of collision energy and system size. Its main goals are to refine studies of the onset of deconfinement and to search for the QCD critical point using fluctuation and correlation observables.

With the interspersion of recollections and personal involvement, this article is both very informative and pleasant to read.  The setup of the experiments and their output of data is presented in 11 figures.   Suggestions of minor amendments before publication are: i) On page 8, line 8 the quantity E_s is introduced. Its meaning  should be explained in the text. ii) The same applies to the inverse slope parameter T appearing in the caption of Fig.6. iii) It is written on page 14, that despite various experimental strategies, no signal has been observed for a critical endpoint in the QCD phase diagram at temperatures around T=150 MeV. This information should also be added to the caption of figure 11 (right) in order not to leave the impression that the sketched red band would be a real feature of the QCD phase diagram. The short line below T= 20 MeV with endpoint CP should be specified as the (well established) liquid-gas phase  transition line of isospin-symmetric nuclear matter.     iv) 4.4. Memories, 6th line: Magda (Kuich) presented a talk entitled...    

Author Response

Comment i):

On page 8, line 8 the quantity E_s is introduced. Its meaning  should be explained in the text:

Response:

The definition of E_S was added.

Comment ii):

The same applies to the inverse slope parameter T appearing in the caption of Fig.6.

Response:

The definition of T was added.

 

Comment iii):

It is written on page 14, that despite various experimental strategies, no signal has been observed for a critical endpoint in the QCD phase diagram at temperatures around T=150 MeV. This information should also be added to the caption of figure 11 (right) in order not to leave the impression that the sketched red band would be a real feature of the QCD phase diagram.

Response:

The information was added to the figure caption. 11

 

Comment iv):

The short line below T= 20 MeV with endpoint CP should be specified as the (well established) liquid-gas phase  transition line of isospin-symmetric nuclear matter.    

Response:

The decribtion was added to caption of Fig. 11

 

Comment v):

4.4. Memories, 6th line: Magda (Kuich) presented a talk entitled...   

Response:

It is modified.

Reviewer 3 Report

Comments and Suggestions for Authors

I suggest a specification in the paper's title indicating the existence of the memories.

At few figures seems to be necessary a better connections among figure's title and text. 

Author Response

Comment i):

I suggest a specification in the paper's title indicating the existence of the memories.

Response:

The title was modified accordingly.

 

Comment ii):

At few figures seems to be necessary a better connections among figure's title and text. 

Response:

It was improved.

Reviewer 4 Report

Comments and Suggestions for Authors

This manuscript presents a historical and reflective overview of quark–gluon plasma (QGP) studies in relativistic heavy-ion collisions at CERN. It traces the quest for QGP from NA35 through NA49-1 and NA49-2, and finally to NA61/SHINE, documenting the evolution of the SPS heavy-ion program and its broader impact on the field. The article is very well organized, with clearly structured sections including an introduction, background, experiment, key results and conclusions, and memories.

The narrative is both scientifically informative and historically valuable. It captures the development of key experimental ideas and observables, particularly the ⟨K⁺⟩/⟨π⁺⟩ ratio, which plays a central role throughout the article. The discussion of how evidence for deconfinement emerged - from early S+S collisions at the SPS in NA35 to the systematic studies in NA49 and NA61/SHINE - is presented in a coherent and compelling manner.

An especially important aspect highlighted in the manuscript is the strategy of mapping the deconfinement condition in the two-dimensional space of atomic number of the ions (A) representing the collision system size and collision energy (√s): keeping the system size large while reducing energy, and conversely maintaining high energy while reducing the system size. This approach provides a conceptual framework for understanding the onset of deconfinement. In this context, the recent O+O and p+O program at the LHC and its exploration of small systems further underscore the continuing relevance of these ideas.

The manuscript also reflects on the broader decades-long evolution of relativistic heavy-ion physics, including the development of the modern nuclear collision phase diagram. As RHIC has now delivered its last collision run, marking the end of an era, the field naturally turns to the question of what the next chapter in high-energy nuclear physics should be. This article provides valuable historical perspective at an important transitional moment for the community.

Overall, the manuscript is clearly written, well structured, and scientifically meaningful. It offers a coherent narrative of the experimental path toward understanding deconfinement in QCD and serves as both a scientific summary and a historical record.

I strongly recommend acceptance of this paper without revision.

Author Response

Comment:

I strongly recommend acceptance of this paper without revision.

Response:

Thanks for the positive evaluation of the manuscript. Minor revisions as requested by other referees were implemented.