REVIEW
Improved nonflow control for multiparticle correlators and system-size-dependent evolution from OO to Pb--Pb collisions at the LHC
Nuclear Experiment · Submitted 27 Oct 2026 · 49 references
Pith's verdict: Solid validation of known nonflow subtraction for new observables; useful AMPT predictions for OO, but the load-bearing LM assumption (no flow in 70-100%) remains untested under realistic conditions.
T0 · AI review
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Referee report
The paper
The paper claims
A v1-based scaling method reliably suppresses nonflow in multiparticle flow correlators for small collision systems, outperforming multiplicity reweighting.
The reading
What carries the argument
Two nonflow subtraction methods anchored on the independent-source picture: (i) multiplicity-reweighting (c0), which scales the low-multiplicity reference correlator by $(N_{m,LM}/N_m)^{m-1}$, and (ii) v1-estimator-based scaling (c1), which uses a nonflow estimator of the same particle order containing at least one power of v1 (e.g., $C_{112}$ for three-particle correlators or $c_1\{4\}$ for four-particle ones). Both methods assume that the low-multiplicity class (70–100%) contains negligible collective flow and that the shape of nonflow correlations does not change with multiplicity. The c1 scaling further assumes that $p_{\text{T}}$-integrated dipolar flow is negligible, allowing v1-based estimators to track the multiplicity dependence of nonflow more accurately.
Extended reading notes
Core claim
The authors find that for the symmetry plane correlators ⟨cos[4(ψ₄-ψ₂)]⟩ and ⟨cos(2ψ₂+3ψ₃-5ψ₅)⟩, and for the normalized symmetric cumulants NSC(4,2) and NSC(3,2), the c1 scaling based on the v1 estimator systematically outperforms the c0 multiplicity-reweighting method in removing nonflow. In a flow-free Monte Carlo generator, c1 yields values consistent with zero for these observables, whereas c0 oversubtracts NSC(4,2) and NSC(3,2) by roughly 20%. The toy model confirms that c1 recovers the flow truth more closely. Applying the c1 subtraction to a multiphase transport model for oxygen-oxygen collisions at √sNN = 5.36 TeV, the correlators show a smooth rise with multiplicity and connect continuously to peripheral lead-lead data from published measurements, demonstrating a system-size evolution from OO to Pb-Pb.
Load-bearing premise
The low-multiplicity class (70–100%) contains no collective flow, and the shape of the nonflow correlation does not change with multiplicity.
Editorial extensions
If this is right
- Future measurements of SPCs and NSCs in OO and Ne-Ne collisions can adopt the c1 subtraction as a standard to extract flow signals free from nonflow bias.
- The system-size comparison from OO to Pb-Pb as a function of ⟨Nch⟩ will reveal whether the same hydrodynamic response describes both systems, testing the universality of collective flow.
- Deviations between transport-model predictions and future data for these observables will provide constraints on initial-state geometry, nuclear structure, and transport properties such as η/s.
- The failure of both methods for the four-particle SPC ⟨cos[6(ψ₆-ψ₂)]⟩ calls for either higher-order estimators or a refined understanding of nonflow in that channel.
Reading between the lines
- The c1 method's reliance on v1 being negligible after pT integration may break down at higher pT or in collisions where dipolar flow is not negligible, limiting its applicability to soft particles (pT ≲ 2 GeV/c).
- If experimental data in OO collisions show a nonzero NSC(3,2) of larger magnitude than the near-zero transport-model prediction, it would suggest that the model underestimates the ε₂-ε₃ anticorrelation in small systems, possibly due to missing nuclear structure effects.
- The smooth OO-to-Pb-Pb transition seen in the results implies that a single scaling variable like ⟨Nch⟩ may organize the system-size dependence of multiparticle correlations, but the mapping between centrality and impact parameter remains weaker in light ions, so comparisons should be interpreted with care.
Original abstract
Multiparticle correlations of event-plane angles, known as symmetry plane correlators (SPCs), and correlations of anisotropic flow amplitudes of different harmonics, known as symmetric cumulants (SCs), help constrain the initial conditions and the transport properties of the system formed in heavy-ion collisions. Such multiparticle correlations are robust against nonflow biases in heavy-ion collisions; however, in small systems the nonflow contribution is sizeable and can distort SPCs and SCs at low multiplicity, mimicking a flow-like signal. Evidence of nuclear-geometry-induced collectivity in light-ion collisions, such as OO and Ne--Ne, has been established at the LHC. Measurements of SPCs and SCs in these systems, and their comparison to heavy-ion collisions, can therefore clarify the system-size dependence of these correlations and the strength of nonlinear mode coupling. In this work, an improved nonflow subtraction framework based on multiplicity reweighting ($\czero$) and $v_1$-estimator-based ($\cone$) scaling is validated for selected SPCs and SCs using a controlled toy model and the \textsc{Pythia~8~Angantyr} model, where collective flow is absent by construction. We find that $\cone$ provides the more reliable nonflow control and apply it to predict the SPCs and normalized SCs in OO collisions at $\sqrt{s_{\rm NN}}=5.36$~TeV with the \textsc{ampt} string-melting model. The results are compared with published Pb--Pb measurements from ALICE and from \textsc{ampt} as a function of midrapidity charged-particle multiplicity $\langle N_{\rm ch}\rangle(|\eta|<0.5)$, placing light- and heavy-ion systems on a common system-size axis.
The compiled record
The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.
The abstract, sentence by sentence
Multiparticle correlations of event-plane angles, known as symmetry plane correlators (SPCs), and correlations of anisotropic flow amplitudes of different harmonics, known as symmetric cumulants (SCs), help constrain the initial conditions and the transport properties of the system formed in heavy-ion collisions. Such multiparticle correlations are robust against nonflow biases in heavy-ion collisions; however, in small systems the nonflow contribution is sizeable and can distort SPCs and SCs at low multiplicity, mimicking a flow-like signal. Evidence of nuclear-geometry-induced collectivity in light-ion collisions, such as OO and Ne--Ne, has been established at the LHC. Measurements of SPCs and SCs in these systems, and their comparison to heavy-ion collisions, can therefore clarify the system-size dependence of these correlations and the strength of nonlinear mode coupling. In this work, an improved nonflow subtraction framework based on multiplicity reweighting ($\czero$) and $v_1$-estimator-based ($\cone$) scaling is validated for selected SPCs and SCs using a controlled toy model and the \textsc{Pythia~8~Angantyr} model, where collective flow is absent by construction. We find that $\cone$ provides the more reliable nonflow control and apply it to predict the SPCs and normalized SCs in OO collisions at $\sqrt{s_{\rm NN}}=5.36$~TeV with the \textsc{ampt} string-melting model. The results are compared with published Pb--Pb measurements from ALICE and from \textsc{ampt} as a function of midrapidity charged-particle multiplicity $\langle N_{\rm ch}\rangle(|\eta|<0.5)$, placing light- and heavy-ion systems on a common system-size axis.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | Multiparticle correlations of event-plane angles, known as symmetry plane correlators (SPCs), and correlations of anisotropic flow amplitudes of different harmonics, known as symmetric cumulants (SCs), help constrain the initial conditions and the transport properties of the system formed in heavy-ion collisions. | background 1.00 | 0.05 | 0.03 | asserted 0.98 |
| s1 | Such multiparticle correlations are robust against nonflow biases in heavy-ion collisions; however, in small systems the nonflow contribution is sizeable and can distort SPCs and SCs at low multiplicity, mimicking a flow-like signal. | background 0.87 | 0.06 | 0.04 | asserted 0.85 |
| s2 | Evidence of nuclear-geometry-induced collectivity in light-ion collisions, such as OO and Ne--Ne, has been established at the LHC. | background 1.00 | 0.08 | 0.03 | asserted 1.00 |
| s3 | Measurements of SPCs and SCs in these systems, and their comparison to heavy-ion collisions, can therefore clarify the system-size dependence of these correlations and the strength of nonlinear mode coupling. | background 0.64 | 0.08 | 0.05 | hedged possibility 0.68 |
| s4 | In this work, an improved nonflow subtraction framework based on multiplicity reweighting ($\czero$) and $v_1$-estimator-based ($\cone$) scaling is validated for selected SPCs and SCs using a controlled toy model and the \textsc{Pythia~8~Angantyr} model, where collective flow is absent by construction. | method 0.79 | 0.95 | 0.10 | asserted 1.00 |
| s5 | We find that $\cone$ provides the more reliable nonflow control and apply it to predict the SPCs and normalized SCs in OO collisions at $\sqrt{s_{\rm NN}}=5.36$~TeV with the \textsc{ampt} string-melting model. | result 0.53 | 0.96 | 0.33 | asserted 1.00 |
| s6 | The results are compared with published Pb--Pb measurements from ALICE and from \textsc{ampt} as a function of midrapidity charged-particle multiplicity $\langle N_{\rm ch}\rangle(|\eta|<0.5)$, placing light- and heavy-ion systems on a common system-size axis. | method 0.46 | 0.80 | 0.19 | asserted 0.96 |
What was examined, and what was not
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Figures
Figures reproduced from arXiv:2610.36922 by the authors.
References
Discussion
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