{"id":"f8d42325-8317-477f-ac7b-367ccf6add5a","arxiv_id":"2608.10578","paper_version":1,"verdict":"CONDITIONAL","confidence":"HIGH","novelty_score":4.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":3,"one_line_summary":"EPOS4 with UrQMD reproduces bulk yields, spectra, pT fluctuations and flow in Pb-Pb and Xe-Xe collisions, and predicts sizable elliptic flow in O-O collisions.","lead":"This study runs the EPOS4 event generator for lead, xenon and oxygen collisions and compares the simulated soft-particle observables with ALICE data, including new predictions for oxygen-oxygen collisions. It supports the view that flow-like collective signals may scale smoothly from large to small collision systems.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Centrality for Xe-Xe and O-O is defined by geometric impact-parameter percentiles rather than by matching ALICE V0 multiplicity classes; if this proxy misassigns small-system centralities, the predicted O-O v2≈0.06 and the claimed continuous scaling are not yet established.","rationale":"The reader's weakest assumption is exactly the one I would identify. The paper compares EPOS4 to ALICE data as a function of centrality, but for Xe-Xe and O-O the centrality classes are defined by geometric b slicing rather than by the V0 multiplicity estimator used by ALICE. Because small systems have large multiplicity fluctuations, this can systematically change which events populate each centrality class, affecting every centrality-differential observable. The O-O prediction v2{2}≈0.06 is quoted for geometric centrality; since v2 rises from central to peripheral events, even a modest broadening of the central bin could bias the prediction upward. The paper explicitly states the proxy is idealized (Section III A), so this is an admitted limitation, but it is not quantified. A single EPOS4-internal re-analysis using a V0-like multiplicity estimator would settle whether the proxy matters. I do not see an internal inconsistency fatal to the paper: the model-data agreement for Pb-Pb and Xe-Xe and the systematic with/without-UrQMD comparisons are real evidence, and the EPOS4 core-corona mechanism is designed to interpolate across system sizes. The other issues (e.g., the ~20% pT-correlator deviations and some contradictory wording about afterburner effects on v2) are either acknowledged or presentation-level. Therefore the reader's CONDITIONAL verdict remains appropriate and no verdict change is needed.","tokens_in":14544,"tokens_out":7815,"duration_ms":67942,"concrete_test":"Within EPOS4, build a V0-like centrality estimator from the simulated charged-particle sum in the V0A and V0C acceptances (2.8<|η|<5.1) and assign Xe-Xe and O-O centrality classes by percentiles of this estimator, using the same binning as ALICE. Recompute the centrality-dependent dN/dη, pT spectra, pT correlator, and v2{2}/v3{2} for Xe-Xe and O-O, and compare with the geometric-b results and with the ALICE Xe-Xe data. If O-O v2{2} shifts by more than ~0.01 in any centrality class, or if the Xe-Xe description worsens beyond the already-reported ~20% deviations, the geometric proxy is the deciding limitation and the continuous-scaling claim must be weakened.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The load-bearing assumption is Section III.A's geometric centrality proxy. For Pb-Pb the b-intervals are tied to published cross-section percentiles, but for Xe-Xe and O-O the centrality classes are obtained by directly slicing the simulated b distribution into geometric percentiles. ALICE centrality is defined by V0 multiplicity percentiles, not by b. In small systems, multiplicity fluctuations are large, so a geometric percentile bin does not contain the same events as the corresponding V0 multiplicity bin. All comparisons (dN/dη, pT spectra, pT correlations, v2/v3) are centrality-differential, so a mismatch would distort apparent agreement and, crucially, the O-O prediction v2{2}≈0.06, because v2 increases from central to peripheral events. The paper explicitly acknowledges this proxy yields 'a narrower and more idealized event sample' for small systems, but never quantifies the bias. Since no O-O data are compared, a prediction quoted in geometric centrality bins is not directly testable against the eventual ALICE V0-based measurement. This makes the geometric proxy the most load-bearing concern; the parameter-transfer assumption is secondary because the same EPOS4 setup is used throughout and is itself part of the model's design.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"This manuscript uses the EPOS4 event generator, with and without the UrQMD hadronic afterburner, to compute charged-particle pseudorapidity densities, identified transverse-momentum spectra, the normalized transverse-momentum correlator, and elliptic/triangular flow harmonics for Pb–Pb collisions at 5.02 TeV, Xe–Xe at 5.44 TeV, and O–O at 5.36 TeV. The Pb–Pb and Xe–Xe results are compared with ALICE data; the O–O results are presented as predictions. The central claim is that EPOS4 provides a consistent, unified description of soft observables from Pb–Pb down to O–O, and predicts a nonzero elliptic flow v2{2} ≈ 0.06–0.07 in O–O collisions, suggesting that collective expansion scales continuously to small systems.","tokens_in":14842,"tokens_out":5306,"duration_ms":45952,"significance":"The paper is a useful model-benchmark contribution: no parameters are fitted in this work, large event samples are generated (0.5M for Pb–Pb and Xe–Xe, 1M for O–O), and the controlled comparison with and without the UrQMD afterburner cleanly exposes the role of late-stage hadronic effects such as baryon-antibaryon annihilation. The O–O prediction is concrete and falsifiable, and the study directly addresses the open question of small-system collectivity. The significance is conditional, however: the Xe–Xe and O–O centrality classes are defined by geometric impact-parameter slicing rather than by the experimental V0-multiplicity selection, and the acknowledged ~20% deviations in the pT correlator and in low-pT flow are not quantified against data uncertainties. If the centrality bias is assessed and the agreement statements are made precise, this would be a solid contribution to the field.","major_comments":[{"comment":"The centrality classes for Xe–Xe and O–O are defined by directly slicing the simulated impact-parameter distribution into geometric percentiles, whereas the ALICE data and the eventual O–O measurement use percentiles of the V0 multiplicity signal. In small systems, where multiplicity fluctuations are large, these two selections need not contain the same events, and the manuscript's own statement that the geometric selection yields 'a narrower and more idealized event sample' indicates a bias that is never quantified. Since every comparison in Figures 1–13 is centrality-differential, and since v2{2} rises strongly from central to peripheral events, a mismatch could distort the apparent agreement and, crucially, the O–O prediction v2{2} ≈ 0.06 in Figure 13. I request a quantitative estimate of the bias: for example, define a V0-like multiplicity estimator in EPOS4 and compare geometric-bin observables with multiplicity-percentile-bin observables for Xe–Xe and O–O, or convert the O–O predictions to V0-multiplicity centrality classes.","section":"Section III A, Figures 1–13"},{"comment":"The text states that the EPOS4 calculation 'show[s] good agreement' with the ALICE pT-correlator data, but in the next sentence acknowledges 'an underestimation of 20% at more central and a similar overestimation towards peripheral region.' A 20% centrality-dependent deviation is a substantial model-data discrepancy that should be discussed explicitly, quantified with respect to the data uncertainties, and propagated into the claim of a 'consistent description' across systems. This is especially relevant because the O–O prediction in the same figure is presented as continuous scaling from the larger systems.","section":"Section III C, Figure 7"},{"comment":"The paper claims that EPOS4 'reproduces the experimental data' and 'successfully describes' the differential flow harmonics, yet the ratio panels in Figure 12 show a suppression for pT ≲ 0.6 GeV/c and a difference of about 20% for pT ≳ 0.6 GeV/c in Pb–Pb and Xe–Xe. Please provide a more precise statement of the level of agreement (e.g., where deviations exceed data uncertainties) and reconcile this with the strong wording. In addition, the sentence 'the hadronic afterburner suppresses the magnitude of v2 at low pT, particularly for kaons and protons. This enhancement is attributed to ...' is internally inconsistent and should be corrected.","section":"Section III D, Figures 9–13"}],"minor_comments":[{"comment":"The sentence 'An underestimation of 20% at more central and a similar overestimation towards peripheral region' is missing a finite verb and should be integrated into the preceding sentence.","section":"Section III C"},{"comment":"The caption contains 'for for Pb–Pb collisions'; the duplicated word should be removed.","section":"Figure 8 caption"},{"comment":"The introduction contains the typo 'behaviuor'; it should read 'behavior' or 'behaviour'.","section":"Section I"},{"comment":"The caption cites reference [45] for both Pb–Pb and Xe–Xe integrated flow, but Pb–Pb data at 5.02 TeV are from reference [44]; please correct the citation.","section":"Figure 13 caption"},{"comment":"The figures use the notation v_n{2} while Eq. (2) defines the scalar-product estimator; please clarify the relation between the two notations (e.g., state explicitly that v_n{2} denotes the scalar-product result in this paper).","section":"Section III D, Eq. (2)"},{"comment":"The text lists centrality classes ending at 50–60%, while the captions of Figures 4 and 5 mention 60–70%; the class definitions should be made consistent across the paper.","section":"Section III A and figure captions"}],"recommendation":"major_revision","confidential_remarks":"The paper fits the journal's scope as a model-data comparison paper. The main technical issue is the geometric centrality proxy for Xe–Xe and O–O; this is load-bearing for the O–O prediction and should be addressed with a quantitative analysis. The citation error for Pb–Pb flow data is minor. I do not see grounds for rejection, but the requested centrality-bias quantification and a more precise statement of the level of agreement are necessary before acceptance."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Here's my take on arXiv:2608.10578. The paper does something worth doing: it runs the same EPOS4 setup across Pb-Pb, Xe-Xe, and O-O, compares to ALICE where data exist, and makes concrete O-O predictions for pT spectra, pT fluctuations, and v2{2}/v3{2}. The central claim—EPOS4 describes soft observables continuously from large to small systems with one parameter set, including a predicted O-O v2{2} around 0.06—is a genuine extrapolation and a useful target for the upcoming O-O run. The with/without UrQMD comparison is also informative, especially for proton yields and low-pT flow.\n\nThe model-data agreement is generally good, and the 20% deviations in the pT correlator and low-pT flow are honestly shown in figures. The paper does not hide its warts.\n\nThe main soft spot is the centrality definition for Xe-Xe and O-O: slicing the simulated b distribution into geometric percentiles instead of matching ALICE's V0 multiplicity percentiles. In small systems this can shift events between centrality bins, and since v2 grows from central to peripheral, the O-O prediction quoted in geometric bins may not be directly testable against the future V0-based measurement. The stress-test note is right that this is the load-bearing concern. The authors acknowledge the idealized event sample in Section III A but never quantify the bias. That is a real caveat, not a fatal one. The parameter transfer from Pb-Pb to O-O is a design choice, not circular reasoning.\n\nI'm a bit less worried than the stress-test note about this sinking the paper. The Pb-Pb and Xe-Xe comparisons are still valid benchmarks in the same binning, and the O-O prediction is clearly labelled as model output in geometric centrality. Still, for the prediction to be properly testable, the paper should either rerun with multiplicity-based centrality or provide a mapping to V0 percentiles. Configuration files would also help reproducibility, though the model is public and run conditions are specified.\n\nBottom line: a serious referee can work with this. It should go to peer review. The centrality question needs to be addressed in revision, but the core analysis is sound and the claims are appropriately cautious.","headline":"Solid three-system EPOS4 benchmark with useful O-O predictions; the geometric centrality proxy for Xe-Xe and O-O is the main caveat but is explicitly acknowledged.","tokens_in":15345,"tokens_out":2332,"would_cite":true,"duration_ms":21072,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"The paper claims that EPOS4, with a single parameter set, describes soft observables from Pb-Pb down to O-O collisions, and predicts an elliptic flow of about 0.06 in oxygen-oxygen collisions, evidence that collective QGP-like expansion…","keywords":["EPOS4","quark-gluon plasma","small-system collectivity","elliptic flow","core-corona separation","oxygen-oxygen collisions","transverse momentum fluctuations","hadronic afterburner"],"falsifier":"Measure the charged-particle elliptic flow $v_{2}\\{2\\}$ in O-O collisions at $\\sqrt{s_{\\mathrm{NN}}} = 5.36$ TeV with ALICE's standard V0 centrality selection and compare with the EPOS4 prediction of $\\approx 0.06$; a significantly smaller value or a different centrality dependence would falsify the scaling claim. Alternatively, re-run the EPOS4 Xe-Xe calculations using multiplicity-based centrality instead of impact-parameter slicing and compare against the published ALICE data to test whether the geometric proxy is the load-bearing ingredient.","tokens_in":14365,"feed_emoji":"⚛️","tokens_out":8645,"duration_ms":66877,"temperature":0.7,"pith_summary":"This paper uses the EPOS4 event generator to compute soft-particle observables — charged-particle multiplicities, identified hadron $p_{\\mathrm{T}}$ spectra, normalized $p_{\\mathrm{T}}$ fluctuations, and elliptic and triangular flow — in lead-lead, xenon-xenon, and oxygen-oxygen collisions at LHC energies. It reports that the model reproduces the ALICE data for Pb-Pb and Xe-Xe with a single parameter set, and predicts that oxygen-oxygen collisions develop a substantial elliptic flow of $v_{2}\\{2\\} \\approx 0.06$–$0.07$, similar in size to mid-central heavy-ion collisions. The paper argues that the continuity of these signatures from the largest to the smallest nuclear system indicates that collective, QGP-like behavior scales smoothly down to small systems. This matters because it gives a concrete, testable prediction linking small-system collisions to the properties of the quark-gluon plasma.","feed_headline":"Oxygen-oxygen collisions predicted to have elliptic flow ≈ 0.06","feed_subtitle":"One EPOS4 parameter set matches Pb-Pb and Xe-Xe data, signaling continuous collectivity down to light nuclei.","key_machinery":"The carrying mechanism is EPOS4's core-corona separation combined with a 3D viscous hydrodynamic evolution of the dense core and microcanonical hadronization, followed by a UrQMD hadronic afterburner. The core-corona picture acts as the scaling device: dense string segments thermalize into a core that expands hydrodynamically and generates radial and anisotropic flow, while dilute segments fragment as corona strings and contribute mostly at high transverse momentum. The model implements event-by-event dynamical saturation scales, which preserve a generalized AGK cancellation so that particle production in the corona region recovers binary scaling at high $p_{\\mathrm{T}}$, while the soft sector is governed by the hydrodynamic response of the core. In this way, the same parameter set, including a shear viscosity-to-entropy ratio $\\eta/s \\approx 0.08$, is carried from Pb-Pb to O-O, making the O-O prediction a direct consequence of the model's internal scaling logic.","core_discovery":"The central claim is that EPOS4 provides a unified, quantitative description of soft observables from Pb-Pb down to O-O at LHC energies, and that the same physics mechanisms operate at all system sizes. In particular, the model predicts a finite and sizable elliptic flow in O-O collisions, $v_{2}\\{2\\} \\approx 0.06$–$0.07$, driven predominantly by event-by-event fluctuations of the initial nucleon positions rather than by the average almond-shaped overlap geometry. The paper further finds that the hadronic afterburner (UrQMD) is essential for reproducing baryon yields through baryon-antibaryon annihilation, and that $p_{\\mathrm{T}}$ fluctuations and flow harmonics scale continuously with system size. The authors present this as evidence that the core-corona separation in EPOS4 — where a hydrodynamically expanding core forms whenever the local density exceeds a threshold — naturally accounts for the transition from large to small systems without changing parameters.","pith_inferences":["The O-O predictions rely on impact-parameter slicing rather than V0-multiplicity centrality, so a direct test would be to compare EPOS4 O-O results against ALICE data selected with the experimental centrality estimator; disagreement in the centrality dependence would reveal where the geometric proxy breaks down.","The same setup could be extended to p-Pb and p-p collisions, and checking whether the predicted O-O v2 interpolates smoothly to measured small-system flow would sharpen the question of whether all small-system collectivity shares one mechanism.","Varying $\\eta/s$ around 0.08 in the O-O runs would show how strongly the predicted $v_{2}$ depends on viscosity; a strong sensitivity would turn the O-O measurement into a direct viscosity constraint for small droplets.","A finer centrality binning and a larger O-O sample could quantify how much of the predicted 10–15% afterburner enhancement of integrated flow is genuinely hadronic, which is a concrete target for experimental systematics."],"forward_implications":["The oxygen-oxygen system is predicted to show a measurable elliptic flow of $v_{2}\\{2\\} \\approx 0.06$–$0.07$, with a weaker centrality dependence than in heavy-ion collisions, so LHC experiments can use O-O runs as a direct test of small-system collectivity.","The success of a single parameter set implies that QGP transport properties, in particular a small $\\eta/s \\approx 0.08$, can be constrained simultaneously by large- and small-system data without ad hoc adjustments.","The hadronic afterburner is required to describe proton yields in all three systems, meaning final-state baryon annihilation is a general feature even in small collisions, and any quantitative model must include it.","The normalized $p_{\\mathrm{T}}$ correlator in O-O follows the same trend as peripheral Pb-Pb/Xe-Xe at similar multiplicities, indicating that initial-state density fluctuations, processed by hydrodynamics, are the common source of momentum correlations across systems.","If the predicted $v_{3}$ in O-O is confirmed, it would corroborate the fluctuation-driven origin of triangular flow and further support a hydrodynamic response in small systems."],"supporting_citations":[{"why":"Supplies the EPOS4 model version and its treatment of flow in small systems; the paper's predicted O-O v2 is a direct output of this framework.","marker":"[28]"},{"why":"Establishes the parallel-scattering, saturation-scale and generalized AGK theorem that controls core-corona particle production and high-pT behavior.","marker":"[26]"},{"why":"Defines the core-corona separation and microcanonical hadronization, the mechanism the paper invokes for continuous system-size scaling.","marker":"[30]"},{"why":"Source of the UrQMD hadronic afterburner that the paper shows is necessary for baryon yields and modifies flow.","marker":"[22]"},{"why":"ALICE Pb-Pb pseudorapidity density data that the model must reproduce for the largest system.","marker":"[34]"},{"why":"ALICE Xe-Xe multiplicity data used as the intermediate-system benchmark.","marker":"[35]"},{"why":"ALICE identified-particle spectra in Pb-Pb, the main dataset for pion, kaon and proton spectral comparisons.","marker":"[36]"},{"why":"ALICE normalized pT correlator data used to validate the fluctuation observable across system sizes.","marker":"[41]"},{"why":"ALICE anisotropic flow data for charged particles in Pb-Pb, the baseline for v2 and v3 comparisons.","marker":"[44]"}],"fun_headline_variants":["Model unifies flow from lead down to oxygen collisions","EPOS4 predicts QGP flow in tiny oxygen collisions","Same hydrodynamics explains Pb to O collisions","Oxygen-oxygen flow predicted at 0.06, model says"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The comparison and predictions depend on defining Xe-Xe and O-O centrality classes by slicing the simulated impact-parameter distribution into geometric percentiles (Section III A) instead of using experimental V0-multiplicity-based selection; if this geometric proxy is biased for small systems, the reported agreement and the O-O predictions could be misleading. The paper also assumes the same EPOS4 parameter set, including $\\eta/s \\approx 0.08$, transfers unchanged from Pb-Pb to O-O.","fun_headline_variants_meta":{"raw":{"variants":["Model unifies flow from lead down to oxygen collisions","EPOS4 predicts QGP flow in tiny oxygen collisions","Same hydrodynamics explains Pb to O collisions","Oxygen-oxygen flow predicted at 0.06, model says"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000454,"raw_usage":{"total_tokens":2397,"prompt_tokens":1173,"completion_tokens":1224,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":789,"completion_tokens_details":{"reasoning_tokens":1158}},"tokens_in":789,"tokens_out":1224,"duration_ms":8961,"temperature":1.0,"reasoning_tokens":1158,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-12T21:30:18.695232+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure the charged-particle elliptic flow $v_{2}\\{2\\}$ in O-O collisions at $\\sqrt{s_{\\mathrm{NN}}} = 5.36$ TeV with ALICE's standard V0 centrality selection and compare with the EPOS4 prediction of $\\approx 0.06$; a significantly smaller value or a different centrality dependence would falsify the scaling claim. Alternatively, re-run the EPOS4 Xe-Xe calculations using multiplicity-based centrality instead of impact-parameter slicing and compare against the published ALICE data to test whether the geometric proxy is the load-bearing ingredient.","supporting_citations":[{"cited_title":"Flow in small systems in the EPOS4 approach for high-energy scatterings","cited_arxiv_id":"2508.07417","evidence_quote":"Supplies the EPOS4 model version and its treatment of flow in small systems; the paper's predicted O-O v2 is a direct output of this framework."}],"review_version":1}