arxiv: 2604.09880 · v2 · submitted 2026-04-10 · ⚛️ nucl-ex · hep-ex

Recognition: unknown

Investigating the onset of deconfinement with NA61/SHINE

Oleksandra Panova (for the NA61/SHINE Collaboration)

Authors on Pith no claims yet

Pith reviewed 2026-05-10 16:50 UTC · model grok-4.3

classification ⚛️ nucl-ex hep-ex

keywords onset of deconfinementheavy-ion collisionskaon to pion ratiohadron productionbaryon transportstrongly interacting matterparticle spectra

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The pith

Measurements of kaon-to-pion ratios in heavy-ion collisions map the onset of deconfinement.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper reports results from a scan that varies both collision energy and the size of the colliding nuclei. It presents kinematic distributions and multiplicities of identified hadrons, with emphasis on the K+/π+ ratio. These data are positioned as essential for locating the point at which strongly interacting matter transitions to a deconfined state. The proton rapidity spectra are examined to trace how baryon number is transported across the scanned range.

Core claim

The obtained results, particularly the K+/π+ ratio, are crucial for understanding the phenomena of the onset of deconfinement. The kinematic distributions and the measured multiplicities of identified hadrons are compared across different systems and energies, while proton rapidity spectra provide a complete picture of the energy and system-size dependence of the mechanism of transport of baryon number.

What carries the argument

The K+/π+ ratio serving as an indicator for the onset of deconfinement within a two-dimensional scan over collision energy and system size.

If this is right

The energy dependence of the K+/π+ ratio can mark the threshold for the onset of deconfinement.
Proton rapidity spectra reveal the energy and system-size dependence of baryon number transport.
Data across different collision systems allow mapping how the onset depends on nuclear size.
Comparisons to earlier measurements confirm the consistency of the observed trends.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

If the ratio exhibits non-monotonic behavior, models of the QCD phase diagram could be tested against it.
Similar scans using other observables such as strange-particle yields might supply independent signals for the transition.

Load-bearing premise

The measured K+/π+ ratio and proton rapidity spectra are assumed to directly signal the onset of deconfinement rather than being shaped primarily by resonance decays or collective flow that change with system size.

What would settle it

A smooth monotonic trend in the K+/π+ ratio with no step or peak across the scanned energies and system sizes would weaken the claim that the ratio marks the onset of deconfinement.

Figures

Figures reproduced from arXiv: 2604.09880 by Oleksandra Panova (for the NA61/SHINE Collaboration).

**Figure 1.** Figure 1: Comparison of rapidity distributions dn/dy of π − mesons produced in central Be+Be, Ar+Sc, Xe+La, and Pb+Pb collisions at √ sNN ≈ 6, 9, and 12 GeV. Statistical uncertainties are shown with error bars, and systematic uncertainties are shown as shaded bands. Published data from Refs. [13, 10, 14, 3]. −4 −2 0 2 4 y 0 20 40 −3 ×10 〉 W 〈 / y /d n d at sNN ≈ 6 GeV + K −4 −2 0 2 4 y 0 20 40 60 −3 ×10 〉 W 〈 / y /d… view at source ↗

**Figure 2.** Figure 2: Comparison of rapidity distributions dn/dy of K+ mesons produced in central Be+Be, Ar+Sc, Xe+La, and Pb+Pb collisions at √ sNN ≈ 6, 9, and 12 GeV. Statistical uncertainties are shown with error bars, and systematic uncertainties are shown as shaded bands. Published data from Refs. [15, 7, 14, 3]. 4. Energy and system size dependence of the strangeness-over-entropy production According to the predictions of… view at source ↗

**Figure 3.** Figure 3: Comparison of rapidity distributions dn/dy of protons produced in central Be+Be, Ar+Sc, and Pb+Pb collisions at √ sNN ≈ 5, 6, 8, 9, 12, and 17 GeV. Statistical uncertainties are shown with error bars, and systematic uncertainties are shown as shaded bands. Published data from Refs. [15, 7, 17, 20]. −2 −1 0 1 2 y 0 0.01 0.02 0.03 0.04 〉0.05 W 〈 / y /d n d Λ at sNN ≈ 6 GeV −2 −1 0 1 2 y 0 0.01 0.02 0.03 0.0… view at source ↗

**Figure 4.** Figure 4: Comparison of rapidity distributions dn/dy of Λ baryons produced in inelastic p+p interactions and central C+C, Si+Si, Ar+Sc, and Pb+Pb collisions at √ sNN ≈ 6, 8, 9, 12, and 17 GeV. Statistical uncertainties are shown with error bars, and systematic uncertainties are shown as shaded bands. Published data from Refs. [11, 21, 22] [PITH_FULL_IMAGE:figures/full_fig_p005_4.png] view at source ↗

**Figure 5.** Figure 5: The energy dependence of the ⟨K+⟩/⟨π +⟩ (left) and K+/π+ (y ≈ 0) (right) for central Be+Be, Ar+Sc, Xe+La, Pb+Pb, and Au+Au collisions as well as inelastic p+p interactions. For Xe+La, y = 0.4 − 0.6 was used as mid-rapidity. For NA61/SHINE points, statistical uncertainties are shown as error bars and systematic as shaded bands. See Ref. [7] for references to published data. An exemplary comparison of the … view at source ↗

**Figure 6.** Figure 6: The energy dependence of the ES for central C+C, Si+Si, Ar+KCl, Ar+Sc, Pb+Pb, and Au+Au collisions as well as inelastic p+p interactions. For NA61/SHINE points, statistical uncertainties are shown as error bars and systematic as shaded bands. See Ref. [30] for references to published data. 1 10 2 10 〈W〉 0.05 0.1 0.15 0.2 0.25 0) ≈ y ( + π / + K p+p Be+Be C+C Ar+Sc Si+Si Xe+La Pb+Pb sNN ≈ 17 GeV =1 s CE HR… view at source ↗

**Figure 7.** Figure 7: The system size dependence of K+/π+ ratios at y ≈ 0 measured at √ sNN ≈ 17 GeV, showing values for inelastic p+p interactions and central Be+Be, C+C, Si+Si, Ar+Sc, Xe+La, and Pb+Pb collisions. The system size is represented by the mean number of wounded nucleons. Statistical uncertainties are shown with bars and systematic uncertainties with square braces. Grey lines represent model simulations. The NA61/S… view at source ↗

read the original abstract

NA61/SHINE is a multipurpose fixed-target experiment located at the CERN SPS. One of its main goals is to study the onset of deconfinement and the properties of strongly interacting matter. For this purpose, a unique two-dimensional scan in collision energy ($\sqrt{s_\mathrm{NN}} = 5.12 - 16.8/17.3$~GeV) and system size (from $p$+$p$ to Pb+Pb) was performed. Results on hadron spectra produced in nucleus-nucleus collisions, including the recent data on charged hadrons produced in central Xe+La collisions and baryons in central Ar+Sc collisions, are presented. The kinematic distributions and the measured multiplicities of identified hadrons are compared with NA49 Pb+Pb results and with available world data. The obtained results, particularly the $K^+/\pi^+$ ratio, are crucial for understanding the phenomena of the onset of deconfinement, which is one of the main aims of the strong interaction program of the NA61/SHINE Collaboration. Additionally, a comparison of proton rapidity spectra in nucleus-nucleus collisions from NA61/SHINE and NA49 is presented, providing a complete picture of the energy and system-size dependence of the mechanism of transport of baryon number at SPS energies.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

This paper adds new data points from NA61/SHINE on Xe+La and Ar+Sc collisions that extend the prior NA49 scan, but the K+/π+ ratio still gets presented as a deconfinement signal without fresh separation from hadronic alternatives.

read the letter

The main takeaway is that NA61/SHINE has released fresh measurements for central Xe+La collisions on charged hadrons and for central Ar+Sc on baryons. These fill gaps in the two-dimensional scan across SPS energies from 5 to 17 GeV and system sizes from p+p up to Pb+Pb. The work shows the kinematic distributions and multiplicities, then lines them up against the older NA49 Pb+Pb results and other world data. The proton rapidity spectra section pulls together the energy and system-size trends in baryon transport, which gives a cleaner overall picture than the previous data set alone. That part is straightforward and useful for anyone modeling stopping at these energies. The paper keeps the focus on the measurements themselves and how they fit the collaboration's long-running program. The K+/π+ ratio receives the usual attention as a potential marker for the onset of deconfinement, and the new points help trace its dependence on system size. The comparisons to NA49 are direct and the data are presented in context, which is the right approach for an experimental report. The soft spot is the continued reliance on the K+/π+ ratio as a key signal without a quantitative handle on competing hadronic effects. Resonance decays, baryon stopping variations, or collective flow can all shift with system size, and the abstract does not show an explicit check that isolates a change in degrees of freedom. The stress-test concern holds here: the claim that these results are crucial rests on the same assumption as earlier NA49 papers, and no new model-independent signature appears in what is described. If the full text has detailed acceptance corrections and systematic breakdowns, that would help, but the interpretation stays within the established framework rather than tightening it. This paper is for people already working on heavy-ion collisions at SPS energies or on the finite-density part of the QCD phase diagram. A reader who needs the latest experimental anchors for models will find the added points worthwhile. It shows honest engagement with the literature and the collaboration's goals, so it deserves a serious referee. I would send it out for peer review so the analysis details and uncertainty treatment can be checked properly.

Referee Report

1 major / 1 minor

Summary. The manuscript reports results from the NA61/SHINE experiment's two-dimensional scan in collision energy (5.12–16.8/17.3 GeV) and system size (p+p to Pb+Pb), including new data on charged hadrons in central Xe+La collisions and baryons in central Ar+Sc collisions. It presents kinematic distributions, multiplicities of identified hadrons, and comparisons to NA49 Pb+Pb results and world data, with emphasis on the K+/π+ ratio and proton rapidity spectra in the context of investigating the onset of deconfinement.

Significance. If the measurements and comparisons hold after full scrutiny of acceptances and systematics, the dataset completes the energy and system-size scan at SPS energies and supplies key reference points for models of hadron production. The inclusion of intermediate systems (Xe+La, Ar+Sc) strengthens the ability to map the evolution of observables such as the K+/π+ ratio across the transition from small to large systems.

major comments (1)

[Abstract] Abstract: The statement that the obtained K+/π+ ratios 'are crucial for understanding the phenomena of the onset of deconfinement' is load-bearing for the paper's motivation, yet the manuscript provides no quantitative separation of the observed energy and system-size dependence from alternative hadronic contributions (resonance decays, collective flow, or baryon stopping) that are known to vary with system size. The comparisons to NA49 and world data alone do not isolate a model-independent signature of deconfinement.

minor comments (1)

[Abstract] The abstract lists the energy range and systems but does not indicate the specific beam energies or centralities for the new Xe+La and Ar+Sc data points; adding these would improve clarity for readers.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the careful review and for recognizing the value of the new Xe+La and Ar+Sc data in completing the NA61/SHINE two-dimensional scan. We address the single major comment below.

read point-by-point responses

Referee: [Abstract] Abstract: The statement that the obtained K+/π+ ratios 'are crucial for understanding the phenomena of the onset of deconfinement' is load-bearing for the paper's motivation, yet the manuscript provides no quantitative separation of the observed energy and system-size dependence from alternative hadronic contributions (resonance decays, collective flow, or baryon stopping) that are known to vary with system size. The comparisons to NA49 and world data alone do not isolate a model-independent signature of deconfinement.

Authors: This manuscript is an experimental data paper whose main purpose is to report the new measurements of charged-hadron spectra in central Xe+La collisions and baryon spectra in central Ar+Sc collisions, thereby completing the energy and system-size scan at SPS energies. The phrasing in the abstract follows the established context of the NA61/SHINE program, in which the K+/π+ ratio has been discussed as a potential probe of the onset of deconfinement on the basis of earlier NA49 Pb+Pb results. The new intermediate-mass systems provide essential additional points that allow the system-size evolution of the ratio to be mapped; such data are required inputs for any quantitative model study that aims to separate deconfinement signals from hadronic effects. The paper itself does not attempt a model-based decomposition, as that lies outside its scope. We are prepared to revise the abstract to state more precisely that the results complete the experimental dataset needed to investigate the onset of deconfinement, rather than asserting that they are crucial in isolation. revision: partial

Circularity Check

0 steps flagged

No circularity: pure experimental measurement paper with direct observables

full rationale

This paper presents measured hadron multiplicities, spectra, and ratios (including K+/π+ and proton rapidity distributions) from a 2D scan in energy and system size at the NA61/SHINE experiment. No derivation chain, theoretical prediction, or first-principles result is claimed that could reduce to fitted inputs, self-definitions, or self-citations. The central statements simply report experimental data and note their relevance to deconfinement studies via comparison to prior NA49 and world data. These comparisons are external benchmarks, not load-bearing justifications for any internal result. The analysis is self-contained as a data report.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

No free parameters, axioms beyond standard relativistic kinematics, or invented entities are introduced; the work consists of direct experimental measurements and comparisons.

pith-pipeline@v0.9.0 · 5535 in / 1149 out tokens · 33093 ms · 2026-05-10T16:50:50.748741+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

29 extracted references

[1]

Abgrallet al

N. Abgrallet al. JINST, vol. 9, P06005, 2014

2014
[2]

Antoniouet al.CERN-SPSC-2006-034, CERN-SPSC-P-330, 2006

N. Antoniouet al.CERN-SPSC-2006-034, CERN-SPSC-P-330, 2006

2006
[3]

Altet al

C. Altet al. PRC, vol. 77, 024903, 2008

2008
[4]

Ga´ zdzicki and M

M. Ga´ zdzicki and M. I. GorensteinAPP B, vol. 30, 2705, 1999

1999
[5]

Aduszkiewiczet al

A. Aduszkiewiczet al. EPJ C, vol. 77, no. 10, 671, 2017

2017
[7]

Adhikaryet al

H. Adhikaryet al. EPJ C, vol. 84, no. 4, 416, 2024

2024
[8]

Abgrallet al

N. Abgrallet al. EPJ C, vol. 74, no. 3, 2794, 2014

2014
[10]

Acharyaet al

A. Acharyaet al. EPJ C, vol. 81, no. 5, 397, 2021

2021
[11]

Aduszkiewiczet al

A. Aduszkiewiczet al. EPJ C, vol. 76, no. 4, 198, 2016

2016
[12]

Bia las, M

A. Bia las, M. Bleszy´ nski, and W. Czy˙ zNPB, vol. 111, no. 3, 461, 1976

1976
[13]

Acharyaet al

A. Acharyaet al. EPJ C, vol. 80, no. 10, 961, 2020

2020
[14]

S. V. Afanasievet al. PRC, vol. 66, 054902, 2002

2002
[15]

Acharyaet al

A. Acharyaet al. EPJ C, vol. 81, no. 1, 73, 2021

2021
[16]

Panova and M

O. Panova and M. LewickiEPJ Web Conf., vol. 274, 05004, 2022

2022
[17]

Anticicet al

T. Anticicet al. PRC, vol. 83, 014901, 2011

2011
[18]

B. B. Backet al. PRL, vol. 86, 1970, 2001

1970
[19]

Ablyazimovet al

T. Ablyazimovet al. EPJ A, vol. 53, no. 3, 2017

2017
[20]

Blume (NA49), private communication

C. Blume (NA49), private communication
[21]

Altet al

C. Altet al. PRC, vol. 78, 034918, 2008

2008
[22]

Altet al

C. Altet al. PRL, vol. 94, 052301, 2005

2005
[23]

B. I. Abelevet al. PRC, vol. 79, 034909, 2009

2009
[24]

Ahleet al

L. Ahleet al. PLB, vol. 476, 1, 2000

2000
[25]

J. L. Klayet al. PRC, vol. 68, 054905, 2003

2003
[26]

Adamczyket al

L. Adamczyket al. PRC, vol. 96, no. 4, 044904, 2017

2017
[27]

Adamet al

J. Adamet al. PRC, vol. 101, no. 2, 024905, 2020

2020
[28]

Acharyaet al

S. Acharyaet al. PRC, vol. 101, no. 4, 044907, 2020

2020
[29]

Abelevet al

B. Abelevet al. PRC, vol. 88, 044910, 2013

2013
[30]

Balkova indico.fais.uj.edu.pl/event/48/contributions/360, 2025

Y. Balkova indico.fais.uj.edu.pl/event/48/contributions/360, 2025

2025
[31]

Ahdidaet al.CERN-PBC Report-2025-002, 2025

C. Ahdidaet al.CERN-PBC Report-2025-002, 2025

2025