Collective effects in O-O and Ne-Ne collisions at sqrt{s_{NN}}=5.36 TeV from a hybrid approach
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The pith
Hybrid hydrodynamic models predict collective flow in O-O and Ne-Ne collisions at the LHC.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
Simulations of O-O collisions with the SMASH-vHLLE hybrid, pure SMASH transport, and Angantyr generate observable predictions that differ according to whether a hydrodynamic quark-gluon plasma stage is included, providing a direct test of collective dynamics in light-ion systems at LHC energies.
What carries the argument
The SMASH-vHLLE hybrid approach, which evolves the dense stage with viscous hydrodynamics and the dilute stage with hadronic transport, allowing side-by-side comparison to pure transport and no-collectivity baselines.
Load-bearing premise
The hybrid framework developed for large heavy-ion systems remains valid when applied to much smaller O-O and Ne-Ne collisions without major retuning.
What would settle it
If measured elliptic flow or other collective observables in O-O collisions at 5.36 TeV match the pure hadronic predictions but deviate from the hybrid results, the applicability of the hydrodynamic stage would be ruled out.
Figures
read the original abstract
Many features of heavy-ion collisions are well described by hybrid approaches, where the droplet of strongly coupled quark gluon plasma (QGP) is modeled by hydrodynamics and the subsequent dilute stage is performed with a hadronic transport model. Conventionally, the formation of a QGP is well established in larger collision systems like lead and gold. However, hints of collectivity were found even in proton-proton collisions, raising the question where the onset of QGP formation lays. This study aims at making predictions for the light-ions run at the CERN Large Hadron Collider in July 2025, in order to explore the applicability of hybrid approaches in smaller collision systems. We employ three different models: the SMASH-vHLLE hybrid approach, the pure hadronic cascade of SMASH, and Angantyr to simulate O-O collisions at a center-of-mass energy of $\sqrt{s_{\mathrm{NN}}}$=5.36 TeV. This setup allows us to compare evolutions with and without a hydrodynamic description on an equal basis, while Angantyr serves as a baseline for no collective effects.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript uses the SMASH-vHLLE hybrid hydrodynamic-transport model, the pure SMASH hadronic cascade, and the Angantyr baseline to generate predictions for collective observables in O-O and Ne-Ne collisions at √s_NN = 5.36 TeV. The central goal is to test the applicability of the hybrid framework (developed for large systems) in smaller collision systems ahead of the 2025 LHC light-ion run by comparing scenarios with and without a hydrodynamic stage.
Significance. If the predictions are borne out, the work supplies concrete, falsifiable forecasts that can help map the onset of QGP-like collectivity in small systems. A clear strength is the equal-footing comparison of three independent models, which isolates the effect of the hydrodynamic evolution without circular reliance on a single framework.
major comments (2)
- [Abstract] Abstract: the claim that the hybrid approach remains valid for O-O and Ne-Ne rests on the unstated assumption that initial conditions, switching hypersurface, and viscosity parameters can be taken unchanged from Pb-Pb settings. No benchmark against existing small-system data (p-Pb or Xe-Xe flow) is reported, which is load-bearing for the central claim that the framework can be applied without major retuning.
- [Model section] Model section (assumed §2): the manuscript does not quantify how the shorter lifetimes and steeper gradients expected in O-O affect the hydrodynamic validity or the switching criterion; this omission directly impacts the reliability of the predicted differences between the hybrid and pure-transport runs.
minor comments (2)
- [Figures] Figure captions should explicitly state whether statistical or systematic uncertainties are shown and how they are estimated for the new predictions.
- [Introduction] A short paragraph comparing the chosen viscosity and switching parameters to those used in prior Pb-Pb studies would improve traceability.
Simulated Author's Rebuttal
We thank the referee for the careful reading and constructive comments on our manuscript. We address each major comment below and have made revisions to the manuscript where appropriate to clarify assumptions and strengthen the discussion.
read point-by-point responses
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Referee: [Abstract] Abstract: the claim that the hybrid approach remains valid for O-O and Ne-Ne rests on the unstated assumption that initial conditions, switching hypersurface, and viscosity parameters can be taken unchanged from Pb-Pb settings. No benchmark against existing small-system data (p-Pb or Xe-Xe flow) is reported, which is load-bearing for the central claim that the framework can be applied without major retuning.
Authors: We agree that the parameters (initial conditions, switching hypersurface, and viscosity) are carried over from our prior Pb-Pb studies without retuning specifically for O-O or Ne-Ne. This choice is intentional to test the framework's applicability in smaller systems on equal footing with the pure-transport and Angantyr runs. In the revised manuscript we have updated the abstract and added an explicit statement of this assumption in the introduction, together with citations to our earlier works that benchmark the same hybrid setup against p-Pb and Xe-Xe flow data. A dedicated retuning study lies outside the scope of the present predictions for the 2025 run. revision: yes
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Referee: [Model section] Model section (assumed §2): the manuscript does not quantify how the shorter lifetimes and steeper gradients expected in O-O affect the hydrodynamic validity or the switching criterion; this omission directly impacts the reliability of the predicted differences between the hybrid and pure-transport runs.
Authors: We have expanded the model section to include estimates of the hydrodynamic lifetime in O-O collisions extracted from the vHLLE evolution and to restate the fixed energy-density switching criterion used. These additions clarify that the criterion is the same as in Pb-Pb but is applied after the system has expanded sufficiently. A quantitative sensitivity analysis of gradient effects on hydrodynamic validity would require additional simulations with varied switching times or viscosities, which we note as a limitation and plan to address in follow-up work; the present equal-footing comparison with pure SMASH already isolates the hydrodynamic contribution. revision: partial
Circularity Check
No significant circularity: predictions from independent model comparison for upcoming light-ion data
full rationale
The paper applies the SMASH-vHLLE hybrid, pure SMASH cascade, and Angantyr (external baseline) to O-O collisions at 5.36 TeV without fitting parameters to the target observables or renaming inputs as predictions. The derivation chain consists of running established models side-by-side to test applicability in smaller systems; no equation reduces to its own fitted inputs by construction, no uniqueness theorem is imported from self-citation to force the framework, and Angantyr supplies an independent no-collectivity reference. The central claim is therefore self-contained against external benchmarks and future data.
Axiom & Free-Parameter Ledger
Lean theorems connected to this paper
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IndisputableMonolith/Cost/FunctionalEquation.leanwashburn_uniqueness_aczel unclear?
unclearRelation between the paper passage and the cited Recognition theorem.
We employ three different models: the SMASH-vHLLE hybrid approach, the pure hadronic cascade of SMASH, and Angantyr to simulate O-O collisions...
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IndisputableMonolith/Foundation/RealityFromDistinction.leanreality_from_one_distinction unclear?
unclearRelation between the paper passage and the cited Recognition theorem.
The switch between a transport description to hydrodynamics relies on the assumption of local thermal equilibrium... opacity γ̂
What do these tags mean?
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- The paper's claim is directly supported by a theorem in the formal canon.
- supports
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- extends
- The paper goes beyond the formal theorem; the theorem is a base layer rather than the whole result.
- uses
- The paper appears to rely on the theorem as machinery.
- contradicts
- The paper's claim conflicts with a theorem or certificate in the canon.
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- Pith found a possible connection, but the passage is too broad, indirect, or ambiguous to say the theorem truly supports the claim.
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Equilibrated fraction of QCD matter in high-energy oxygen--oxygen collisions
The equilibrated core in O+O collisions overtakes the nonequilibrium corona above midrapidity multiplicity of about 20, yet corona contributions persist in central events, making pure hydrodynamics inadequate.
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