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arxiv: 2606.22558 · v1 · pith:X7KSUGI5new · submitted 2026-06-21 · ⚛️ nucl-th · hep-ph

Event-by-event fluctuations of elliptic flow in ultrarelativistic O+O collisions

Renata Krupczak , Nicolas Borghini , Hendrik Roch This is my paper

Pith reviewed 2026-06-26 09:37 UTC · model grok-4.3

classification ⚛️ nucl-th hep-ph
keywords elliptic flowO+O collisionsevent-by-event fluctuationsinitial eccentricityhydrodynamic modelcentrality dependenceultrarelativistic collisions
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The pith

In O+O collisions, elliptic flow and initial eccentricity arise mainly from event-by-event fluctuations in energy deposition rather than average geometry.

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

The paper models oxygen-oxygen collisions at LHC energies with a three-dimensional hydrodynamic approach that matches observed charged-hadron multiplicities versus centrality and pseudorapidity. It establishes that the initial elliptical eccentricity is set primarily by random variations in energy deposition, producing large event-by-event spreads even inside one centrality bin. The same fluctuation origin governs elliptic flow v2, setting O+O apart from heavy-nucleus collisions where geometric overlap dominates. A decomposition of initial conditions into an average profile plus uncorrelated modes shows that only a small number of modes suffice to recover the joint distributions of eccentricity and flow.

Core claim

In ultrarelativistic O+O collisions the initial elliptical eccentricity is mainly driven by the fluctuations of the energy deposition and thereby varies considerably event-by-event within a fixed centrality class. This also holds for elliptic flow v2, whose origin in O+O thus differs from that in collisions of heavy nuclei. Using a decomposition of initial states in an average event and uncorrelated modes, the joint probability distribution of eccentricity and elliptic flow can be reproduced with reasonable accuracy with only a small set of fluctuation modes.

What carries the argument

Decomposition of initial states into an average event plus uncorrelated fluctuation modes within the McDipper+MUSIC hydrodynamic framework.

If this is right

  • Elliptic flow in O+O collisions must be treated as fluctuation-dominated rather than geometry-dominated.
  • Centrality-selected samples in small systems contain substantial intrinsic variation in eccentricity and flow.
  • A limited set of uncorrelated modes captures the essential statistics of eccentricity and v2 despite large fluctuations.
  • Models of small-system collectivity need to incorporate detailed energy-deposition fluctuations to match data.

Where Pith is reading between the lines

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

  • Similar fluctuation dominance is expected in other light-ion systems such as p+O or Ne+Ne.
  • Hydrodynamic codes for small collisions require event-by-event initial conditions rather than averaged profiles.
  • Interpreting flow observables in small systems may shift emphasis from initial geometry to stochastic deposition patterns.

Load-bearing premise

The McDipper+MUSIC model accurately reproduces the measured charged-hadron multiplicity dependence on centrality and pseudorapidity.

What would settle it

An experimental measurement of the event-by-event variance or joint distribution of v2 in O+O collisions that deviates significantly from the model's fluctuation-driven prediction.

Figures

Figures reproduced from arXiv: 2606.22558 by Hendrik Roch, Nicolas Borghini, Renata Krupczak.

Figure 1
Figure 1. Figure 1: FIG. 1. Centrality dependence of the charged-hadron multi [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. Pseudorapidity dependence of the charged-hadron [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3. Energy profile for the average initial state in O+O collisions at [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4. Spacetime rapidity dependence of the coordinates [PITH_FULL_IMAGE:figures/full_fig_p004_4.png] view at source ↗
Figure 6
Figure 6. Figure 6: FIG. 6. Transverse energy profile at [PITH_FULL_IMAGE:figures/full_fig_p005_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: FIG. 7. Normalized profiles of the first 10 fluctuation modes Ψ [PITH_FULL_IMAGE:figures/full_fig_p006_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: FIG. 8. Same as Fig. 7 for the first five fluctuation modes (from top to bottom: [PITH_FULL_IMAGE:figures/full_fig_p007_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: FIG. 9. Same as Fig. 8 for the first five fluctuation modes (from top to bottom: [PITH_FULL_IMAGE:figures/full_fig_p008_9.png] view at source ↗
Figure 10
Figure 10. Figure 10: FIG. 10. Relative weights (6) of the average initial state [PITH_FULL_IMAGE:figures/full_fig_p008_10.png] view at source ↗
Figure 11
Figure 11. Figure 11: FIG. 11. Joint probability distribution of [PITH_FULL_IMAGE:figures/full_fig_p010_11.png] view at source ↗
Figure 13
Figure 13. Figure 13: FIG. 13. Longitudinal decorrelation of [PITH_FULL_IMAGE:figures/full_fig_p012_13.png] view at source ↗
read the original abstract

We study O+O collisions at $\sqrt{s_\mathrm{NN}} = 5.36$ TeV within a fully three-dimensional $\text{McDipper}$+$\text{MUSIC}$ model, which allows us to describe the experimentally measured dependence of charged hadron multiplicity on centrality and pseudorapidity. We show that the initial elliptical eccentricity is mainly driven by the fluctuations of the energy deposition and thereby varies considerably event-by-event within a fixed centrality class. This also holds for elliptic flow $v_2$, whose origin in O+O thus differs from that in collisions of heavy nuclei. Using a decomposition of initial states in an average event and uncorrelated modes, we find that despite the large size of fluctuations we can reproduce the joint probability distribution of eccentricity and elliptic flow with a reasonable accuracy with only a small set of fluctuation modes.

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

0 major / 2 minor

Summary. The manuscript studies O+O collisions at √s_NN = 5.36 TeV with the McDipper+MUSIC model, which is stated to reproduce the measured charged-hadron multiplicity dependence on centrality and pseudorapidity. It argues that initial elliptical eccentricity is driven primarily by fluctuations in energy deposition and therefore fluctuates strongly event-by-event even inside a fixed centrality bin; the same holds for elliptic flow v2, implying a fluctuation-dominated origin unlike the geometry-driven case in heavy-ion collisions. A decomposition of initial states into an average event plus uncorrelated modes is shown to reproduce the joint probability distribution of eccentricity and v2 to reasonable accuracy with only a small set of modes.

Significance. If the model validation and decomposition accuracy hold, the work clarifies that elliptic flow in light systems such as O+O arises from energy-deposition fluctuations rather than participant geometry, offering a concrete distinction from heavy-ion phenomenology. The mode-decomposition technique is a constructive element, as it demonstrates that a limited number of uncorrelated modes can capture the joint distribution despite large fluctuations.

minor comments (2)
  1. [Abstract] The abstract asserts that McDipper+MUSIC 'allows us to describe' the multiplicity data; explicit quantitative comparisons (e.g., χ² values or overlaid plots versus centrality and η) should be referenced by figure or table number to substantiate this central modeling assumption.
  2. The phrase 'reasonable accuracy' for the mode decomposition is used without a numerical metric; a quantitative measure (e.g., overlap integral or Kolmogorov-Smirnov distance between the reconstructed and full distributions) would clarify the claim that a small set of modes suffices.

Simulated Author's Rebuttal

0 responses · 0 unresolved

We thank the referee for their positive summary, significance assessment, and recommendation of minor revision. No specific major comments appear in the report.

Circularity Check

0 steps flagged

No significant circularity identified

full rationale

The paper performs forward hydrodynamic simulations with the established McDipper+MUSIC model, first validating that the model reproduces measured charged-hadron multiplicity vs. centrality and pseudorapidity, then extracting eccentricity and v2 distributions directly from the generated events. The central claim that fluctuations dominate elliptical eccentricity (and thus v2) within a fixed centrality class follows as a post-processing observation on the simulated initial states and final flow; it is not obtained by fitting those quantities to data or by any self-referential definition. No load-bearing step reduces to a fitted input renamed as prediction, a self-citation chain, or an ansatz smuggled via prior work. The derivation chain is therefore self-contained against external multiplicity benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claim rests on the hydrodynamic model's ability to map initial energy fluctuations to final flow and on the validity of the average-plus-uncorrelated-modes decomposition.

axioms (1)
  • domain assumption McDipper+MUSIC reproduces the measured multiplicity dependence on centrality and pseudorapidity in O+O collisions.
    Explicitly invoked in the abstract as the justification for using the model.

pith-pipeline@v0.9.1-grok · 5672 in / 1107 out tokens · 20831 ms · 2026-06-26T09:37:15.867465+00:00 · methodology

discussion (0)

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