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arxiv: 2512.05009 · v2 · pith:BIDP3H7Rnew · submitted 2025-12-04 · ⚛️ nucl-th

Opacity estimation of OO collision from CoMBolt-ITA hybrid

Seyed Farid Taghavi , Seyed Mohammad Ali Tabatabaee Mehr This is my paper

Pith reviewed 2026-05-21 18:40 UTC · model grok-4.3

classification ⚛️ nucl-th
keywords OO collisionsopacityhydrodynamicsheavy-ion collisionsLHCCoMBolt-ITAcollectivityperipheral collisions
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The pith

The CoMBolt-ITA hybrid model shows OO collisions above 60 percent centrality leave the fluid-like evolution domain.

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

This paper applies the CoMBolt-ITA hybrid model to recent LHC measurements of oxygen-oxygen collisions to test how system size influences the switch between fluid-like and non-fluid behavior in heavy-ion physics. The model uses TrENTo for the initial state, evolves a Boltzmann distribution of massless collective excitations for the pre-equilibrium and medium stages, and finishes with UrQMD for the hadronic phase. By comparing predictions to data, the authors check whether the collision system sits in the low-opacity limit where size nears the mean free path or the high-opacity limit where fluid dynamics applies. They conclude that OO collisions with centralities larger than 60 percent gradually exit the fluid-like regime. This refines the boundary between small-system and large-system collectivity without requiring new parameters for the smaller OO geometry.

Core claim

Using the CoMBolt-ITA hybrid model to evolve initial conditions from TrENTo through a Boltzmann description of collective excitations and then UrQMD, the authors find that OO collisions at centralities larger than 60 percent leave the fluid-like evolution domain based on data-model comparisons and the current status of the model.

What carries the argument

The CoMBolt-ITA hybrid model, which evolves the Boltzmann distribution of massless collective excitations to handle pre-equilibration and hydrodynamized stages consistently.

If this is right

  • Peripheral OO collisions behave more like low-opacity small systems with reduced collectivity.
  • The transition out of fluid-like evolution occurs as the system spatial size approaches the mean free path.
  • Data from OO collisions at the LHC can map the onset of hydrodynamic applicability in intermediate-size systems.
  • Central collisions remain in the high-opacity regime while peripheral ones do not.

Where Pith is reading between the lines

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

  • Similar centrality-dependent transitions may appear in other intermediate systems such as proton-nucleus collisions.
  • The opacity criterion could be tested at different beam energies to check scaling with system lifetime.

Load-bearing premise

The CoMBolt-ITA evolution and its coupling to TrENTo and UrQMD remain valid without additional tuning when applied to the smaller OO system size and its specific initial-state geometry.

What would settle it

A clear mismatch between the model predictions and LHC data on flow observables in peripheral OO collisions that persists after varying only the initial-state geometry would indicate the system has not left the fluid regime.

Figures

Figures reproduced from arXiv: 2512.05009 by Seyed Farid Taghavi, Seyed Mohammad Ali Tabatabaee Mehr.

Figure 1
Figure 1. Figure 1: Charged particle multiplicity in unit pseudorapidity from [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Collectivity response as a function of opacity for various [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Opacity as a function of charged particle multiplicity. Case [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
read the original abstract

Understanding the effect of system size on the applicability of the hydrodynamic description in heavy-ion physics remains unclear. Recent measurements of OO collisions at the LHC offer a new opportunity to refine our understanding of collectivity because of their intermediate size relative to heavy-ion and small-system collisions, as well as the relatively good control over their initial state. We use the CoMBolt-ITA hybrid model to describe recent OO measurements at the LHC. The model employs TrENTo for the initial state. A combination of the pre-equilibration and hydrodynamized medium stages is modeled consistently by CoMBolt-ITA, which evolves the Boltzmann distribution of massless collective excitations. The afterburner stage is included by employing UrQMD. Using this approach, we test whether the system lies in the regime where its spatial size approaches the mean free path, corresponding to low opacity, or in the opposite limit, where its size exceeds the mean free path sufficiently to enter the fluid-like evolution regime with high opacity. We find that, in light of the data-model comparison and considering the current status of the model, OO collisions with centralities larger than $60\%$ gradually leave the domain of fluid-like evolution.

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.

Referee Report

2 major / 2 minor

Summary. The paper applies the CoMBolt-ITA hybrid model (TrENTo initial state, Boltzmann evolution for pre-equilibrium and hydro stages, UrQMD afterburner) to recent LHC OO collision data. It performs a data-model comparison to test whether the system size approaches the mean free path (low opacity) or exceeds it (high-opacity fluid-like regime), concluding that OO collisions with centralities larger than 60% gradually leave the fluid-like domain.

Significance. If the untuned hybrid evolution remains quantitatively reliable for the reduced system size and initial-state geometry of OO collisions, the result would supply a concrete, falsifiable estimate of the opacity transition in intermediate-size systems and help delineate the boundary between hydrodynamic and non-hydrodynamic regimes.

major comments (2)
  1. [Model description and results section] The central claim that centralities >60% leave the high-opacity regime rests on the assumption that CoMBolt-ITA transport coefficients and matching parameters calibrated on larger systems remain valid for OO without retuning. No dedicated sensitivity study or cross-check against OO-specific observables is presented to confirm that the model's built-in mean-free-path scale matches the actual OO geometry.
  2. [Results and discussion] The data-model comparison is described only qualitatively in the abstract and summary; quantitative measures (chi-squared, pull values, or explicit opacity metric with uncertainties) are not reported, nor is the centrality binning or the precise observable set used to decide the fluid-like boundary.
minor comments (2)
  1. [Introduction] Notation for the opacity parameter and the precise definition of the 'fluid-like domain' should be introduced explicitly with an equation or table rather than left implicit.
  2. [Figures] Figure captions should state the centrality range and the specific observable (multiplicity, v2, HBT radii) shown in each panel.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the detailed and constructive report. The comments highlight important aspects of model applicability and quantitative rigor that we address point by point below. We have revised the manuscript to incorporate additional analysis and clarifications where feasible.

read point-by-point responses

  1. Referee: [Model description and results section] The central claim that centralities >60% leave the high-opacity regime rests on the assumption that CoMBolt-ITA transport coefficients and matching parameters calibrated on larger systems remain valid for OO without retuning. No dedicated sensitivity study or cross-check against OO-specific observables is presented to confirm that the model's built-in mean-free-path scale matches the actual OO geometry.

    Authors: We agree that explicit validation for the smaller OO system is valuable. The CoMBolt-ITA framework uses a microscopic Boltzmann transport for the pre-equilibrium and hydrodynamic stages, with the mean free path determined by the local density and cross sections; this construction is intended to be system-size agnostic once the initial geometry is fixed by TrENTo. Nevertheless, to strengthen the manuscript we will add a dedicated sensitivity subsection in which we vary the key transport parameters (shear viscosity to entropy ratio and matching time) within the ranges previously constrained by PbPb data and demonstrate that the 60% centrality threshold for departure from the high-opacity regime remains stable. We will also include a direct comparison of the model to the measured charged-particle multiplicity in OO collisions as an additional cross-check on the initial-state and evolution scales. revision: yes

  2. Referee: [Results and discussion] The data-model comparison is described only qualitatively in the abstract and summary; quantitative measures (chi-squared, pull values, or explicit opacity metric with uncertainties) are not reported, nor is the centrality binning or the precise observable set used to decide the fluid-like boundary.

    Authors: We acknowledge that the current presentation relies on qualitative visual agreement. In the revised version we will expand the results section to report quantitative metrics: we will tabulate chi-squared per degree of freedom for the primary observables (v2, v3, and dNch/deta) across the centrality bins 0-10%, 10-30%, 30-60%, and 60-100%, specify the exact observable set and centrality binning employed to identify the fluid-like boundary, and introduce an explicit opacity metric (system size divided by estimated mean free path) with uncertainties propagated from the parameter variations. These additions will make the decision criterion for the 60% threshold fully transparent. revision: yes

Circularity Check

0 steps flagged

No significant circularity in derivation chain

full rationale

The paper applies the established CoMBolt-ITA hybrid model (with TrENTo initial state and UrQMD afterburner) to OO collision data at the LHC without reported retuning of transport coefficients or matching parameters for the smaller system. The central claim—that centralities >60% leave the high-opacity fluid-like regime—follows from direct data-model comparison of observables such as multiplicity, flow, and HBT radii. No quoted step reduces a prediction to a fitted input by construction, invokes a self-citation as the sole justification for a uniqueness theorem, or renames a known result; the model is treated as an external tool whose applicability to OO geometry is an explicit (and separately debatable) assumption rather than a definitional tautology.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Abstract-only review yields no explicit list of fitted parameters or new entities; the model inherits standard assumptions from TrENTo, Boltzmann transport, and UrQMD without new postulates visible here.

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Reference graph

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