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Study of the multiplicity dependence of non-flow correlations in pp collisions at $\sqrt{s}=200$ GeV using PYTHIA

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Milan Stojanovic, Joern Putschke

Nuclear Theory · Submitted 10 Sep 2026 · 43 references

Pith's verdict: A careful PYTHIA study showing that non-flow scaling assumptions fail at RHIC energies, undermining standard subtraction methods—but the conclusions are model-dependent.

T0 · AI review

How this review was made: An AI referee (deepseek/deepseek-v4-flash) read the paper on September 11, 2026 and answered a fixed list of questions about it. T0 is the starting level every paper gets. The levels describe how deeply Pith has checked the paper, never the authors.

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  3. T2 Compute The paper's code or calculations re-run with the same results.
  4. T3 Formal statement The claims written out in a formal math language a computer can read.
  5. T4 Proof Those claims proved, and the proof checked by computer.

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arXiv ID2609.11526 (version 1)
Pith IDFTK4YE6F, Pith's permanent record of this paper and its review
CitationJoern Putschke, Milan Stojanovic. "Study of the multiplicity dependence of non-flow correlations in pp collisions at $\sqrt{s}=200$ GeV using PYTHIA." arXiv:2609.11526 (2026). https://pith.science/paper/FTK4YE6F
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The paper

The paper claims

Non-flow correlations in pp at RHIC do not scale simply with event multiplicity

The reading

This paper uses PYTHIA simulations of proton-proton collisions at RHIC energy 200 GeV to test a key assumption behind standard non-flow subtraction methods: that jet-induced correlations scale universally with event multiplicity. The authors show that the Fourier coefficients V1Δ, V2Δ, and V3Δ, extracted from two-particle azimuthal correlations with a pseudorapidity gap, evolve differently with multiplicity. This breakdown persists even when using forward or underlying-event multiplicity estimators to reduce phase-space overlap. The cause is traced to a multiplicity-selection bias that systematically favorsevents with harder jets and different fragmentation patterns. If confirmed in data, these findings imply that the low-multiplicity subtraction and template fit methodsused at the LHC cannot be directly applied at RHIC energies without significant corrections.

What carries the argument

The key tool is the two-particle azimuthal correlation function with a pseudorapidity gap |Δη|>2, Fourier-decomposed into coefficients VnΔ. These coefficients are compared across multiplicity intervals to test the scaling assumption. The infrared regularization scale pT0 in PYTHIA is varied to probe the hard-soft interplay.

Extended reading notes

Core claim

The central claim is that non-flow contributions in pp collisions at √s = 200 GeV do not follow a simple, harmonic-independent scaling with event multiplicity, as assumed by the low-multiplicity subtraction and template fit methods. This is demonstrated by the multiplicity evolution of V1Δ, V2Δ and V3Δ normalized to their lowest-multiplicity values: the ratios for different n deviate from a common trend, and the deviation issensitive to the relative balance of hard and soft processes (controlled by the PYTHIA pT0 parameter). Additionally, the jet cross-section and jet fragmentation patterns show strong multiplicity dependence, confirming that multiplicity selection biases the sample toward harder, more fragmented jets.

Load-bearing premise

The conclusions depend on PYTHIA 8.314 providing a realistic description of jet production and fragmentation in pp collisions at 200 GeV; if PYTHIA misrepresents the multiplicity dependence of hard parton production, the central claim may not hold in real data.

Editorial extensions

If this is right

  • Existing RHIC measurements of collective flow in small systems that rely on standard non-flow subtraction should be re-evaluated with a quantitative assessment of this bias.
  • A practical data-driven cross-check is recommended: experiments should examine the multiplicity dependence of jet cross-section and fragmentation before applying any non-flow subtraction.
  • The finding suggests that at lower collision energies the interplay between hard and soft processes is more complex, and LHC-developed methods cannot be transferred without validation.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • If experimental data from STAR confirm the non-uniform scaling, the community may need to adopt alternative non-flow rejection methods (e.g., sub-event cumulants or jet-veto) that do not assume a simple scaling.
  • The selection bias demonstrated here may also affect measurements of jet modification and underlying-event observables in small systems at RHIC, beyond just non-flow correlations.
Original abstract

The observation of a near-side enhancement in long-range correlations within high-multiplicity pA and pp collisions, commonly referred to as the ridge, has motivated significant efforts to quantify possible collective effects in small systems. Estimation of non-flow contributions in two-particle correlation measurements in small systems at the LHC typically relies on the assumption that jet-induced non-flow correlations are independent of event multiplicity. Consequently, this assumption allows the high-multiplicity non-flow to be estimated and accounted for by directly using low-multiplicity events. In this paper, the validity of this assumption is studied examining the multiplicity dependence of non-flow contributions estimated via two-particle correlation functions in pp collisions at RHIC energies $\sqrt{s} = 200$~GeV using the PYTHIA event generator. It is observed that selecting any specific multiplicity range inherently biases jet production dynamics. These biases introduce a non-trivial multiplicity dependence of the non-flow contribution at RHIC energies. The dependence of jet fragmentation and $p_{\text{T}}$-differential cross sections on event multiplicity is studied using a variety of commonly used multiplicity estimators, including mid-rapidity charged-particle multiplicity, forward multiplicity, and underlying-event activity. The results indicate that non-flow contributions do not exhibit simple multiplicity scaling and challenge the applicability of standard subtraction procedures utilized at LHC energies at lower RHIC collision energies.

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.

Compiled by classifier-1.13.0 via TypeSafe API, question bank abstract-p1-20260923, 2026-09-23: 1 request, 6,470 input tokens.

What the abstract itself states, as probabilities: claims first 0.04 · states limitation 0.46 · human participants 0.01 · controlled comparison 0.23 · randomized assignment 0.02 · states quantitative result 0.05 · compares with prior results 0.68 · mentions code or data availability 0.02.

The abstract, sentence by sentence

arXiv listing abstract v1. Results and the main claim are underlined; .

The observation of a near-side enhancement in long-range correlations within high-multiplicity pA and pp collisions, commonly referred to as the ridge, has motivated significant efforts to quantify possible collective effects in small systems. Estimation of non-flow contributions in two-particle correlation measurements in small systems at the LHC typically relies on the assumption that jet-induced non-flow correlations are independent of event multiplicity. Consequently, this assumption allows the high-multiplicity non-flow to be estimated and accounted for by directly using low-multiplicity events. In this paper, the validity of this assumption is studied examining the multiplicity dependence of non-flow contributions estimated via two-particle correlation functions in pp collisions at RHIC energies $\sqrt{s} = 200$~GeV using the PYTHIA event generator. It is observed that selecting any specific multiplicity range inherently biases jet production dynamics. These biases introduce a non-trivial multiplicity dependence of the non-flow contribution at RHIC energies. The dependence of jet fragmentation and $p_{\text{T}}$-differential cross sections on event multiplicity is studied using a variety of commonly used multiplicity estimators, including mid-rapidity charged-particle multiplicity, forward multiplicity, and underlying-event activity. The results indicate that non-flow contributions do not exhibit simple multiplicity scaling and challenge the applicability of standard subtraction procedures utilized at LHC energies at lower RHIC collision energies.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#sentencerolethe authors' own workstates the main conclusionstrength
s0 The observation of a near-side enhancement in long-range correlations within high-multiplicity pA and pp collisions, commonly referred to as the ridge, has motivated significant efforts to quantify possible collective effects in small systems. background 1.00 0.05 0.03 asserted 0.99
s1 Estimation of non-flow contributions in two-particle correlation measurements in small systems at the LHC typically relies on the assumption that jet-induced non-flow correlations are independent of event multiplicity. background 0.98 0.06 0.03 asserted 0.99
s2 Consequently, this assumption allows the high-multiplicity non-flow to be estimated and accounted for by directly using low-multiplicity events. background 0.97 0.06 0.03 asserted 0.96
s3 In this paper, the validity of this assumption is studied examining the multiplicity dependence of non-flow contributions estimated via two-particle correlation functions in pp collisions at RHIC energies $\sqrt{s} = 200$~GeV using the PYTHIA event generator. contribution 0.96 0.82 0.05 asserted 0.72
s4 It is observed that selecting any specific multiplicity range inherently biases jet production dynamics. result 0.98 0.83 0.13 asserted 1.00
s5 These biases introduce a non-trivial multiplicity dependence of the non-flow contribution at RHIC energies. result 0.96 0.88 0.31 asserted 1.00
s6 The dependence of jet fragmentation and $p_{\text{T}}$-differential cross sections on event multiplicity is studied using a variety of commonly used multiplicity estimators, including mid-rapidity charged-particle multiplicity, forward multiplicity, and underlying-event activity. method 0.92 0.88 0.04 asserted 0.94
s7 The results indicate that non-flow contributions do not exhibit simple multiplicity scaling and challenge the applicability of standard subtraction procedures utilized at LHC energies at lower RHIC collision energies. interpretation 0.57 0.91 0.87 hedged possibility 0.74

What kind of paper the classifier reports this is: simulation study 1.00 · evidence from simulation or computation 1.00.

What was examined, and what was not

sources read
  • arXiv listing abstract v1
not examined
  • full text
  • figures and tables
  • references and citing sentences
  • relations to other papers
levels run
  • P abstract
not run
  • S sections
  • T body sentences
  • C citations
  • R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

Figures
Figure 1. FIG. 1: Illustration of the three event multiplicity estimators considered in this work, shown for the same event.
Figure 2. FIG. 2: Correlation between event multiplicities
Figure 3. FIG. 3: Charged-particle multiplicity distributions for
Figure 5. FIG. 5: Azimuthal anisotropy coefficients
Figure 6. FIG. 6: Azimuthal anisotropy coefficients
Figure 7. FIG. 7: Impact of the infrared regularization scale
Figure 9. FIG. 9: Jet multiplicity distribution (for
Figure 12. FIG. 12: The
Figure 13. FIG. 13: Jet multiplicity distribution (for
Figure 14. FIG. 14: Two-particle azimuthal correlation function as
Figure 15. FIG. 15: Two-particle azimuthal correlation function as
Figure 16. FIG. 16: Two-particle azimuthal correlation function as
Figure 18. FIG. 18: Two-particle azimuthal correlation function as
Figure 19. FIG. 19: Charged-particle multiplicity distributions

Showing 14 of the paper's 19 figures. See all figures in the PDF ↗ Figures reproduced from arXiv:2609.11526 by the authors.

References

Discussion

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