{"id":"a51dff8b-0bb2-46ee-afa8-d631272b2a26","arxiv_id":"2602.19566","paper_version":2,"verdict":"CONDITIONAL","confidence":"HIGH","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":4,"one_line_summary":"In PYTHIA 8 pp collisions at 13 TeV, the highest-RT events develop a long-range near-side ridge in R2^CI, while P2^CI and charge-dependent correlators do not.","lead":"Simulations of proton-proton collisions with PYTHIA 8 show a ridge-like long-range correlation in the charge-independent two-particle number correlator for events with the most underlying-event activity, while charge-dependent and transverse-momentum correlators stay localized. This gives experimentalists a non-hydrodynamic reference pattern to compare against LHC small-system 'collectivity' signals.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Ridge in R2^CI may be a high-multiplicity selection effect rather than UE-specific; no N_ch-matched control or MPI/CR variation is shown.","rationale":"The reader's weakest assumption correctly identifies the lack of a multiplicity-matched control and the untested MPI/CR attribution. This is the most load-bearing concern because the central claim is not merely that PYTHIA produces a ridge—it is that the ridge is caused by enhanced UE activity specifically. The observed correlation between R_T and N_ch (Fig. 3) means the ridge could be a trivial consequence of selecting high-multiplicity events. If that were the case, the paper would not establish R_T as a probe of UE-specific dynamics. The absence of statistical uncertainties and the unspecified ZYAM procedure are additional weaknesses, but they are secondary to the missing control. The simulation result itself is plausible and worth reporting, so a REJECT is not warranted; however, the interpretation should be conditional on the proposed control. Since the reader's verdict already is CONDITIONAL, my assessment does not change the verdict.","tokens_in":13769,"tokens_out":4664,"duration_ms":47272,"concrete_test":"Run PYTHIA 8 (same Monash tune and event selection) and bin events in the (R_T, N_ch) plane. Compare the R2^CI Δφ projections at |Δη|>1.2 for events in the highest R_T class (2.5<R_T≤5.0) against events in lower R_T classes but with matched N_ch distributions (e.g., by reweighting or narrow N_ch bins). If the long-range near-side yield disappears or becomes comparable when N_ch is matched, the claimed UE-specific ridge is actually a multiplicity effect. As a secondary check, rerun with ColorReconnection:mode=0 or MPI disabled to test the specific MPI/CR attribution.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central observation—that a long-range near-side component appears in R2^CI only for 2.5<R_T≤5.0—is used to conclude that enhanced underlying-event activity (and specifically MPI+CR) produces collectivity-like correlations. However, Fig. 3 shows that N_ch increases monotonically with R_T, so the highest R_T class is also the highest-multiplicity class. The paper provides no multiplicity-matched control; hence the ridge could simply be a high-multiplicity selection effect, not a UE-specific effect. This matters because R_T is defined via the transverse-region multiplicity N_T, which is correlated with the total multiplicity. Without separating R_T from N_ch, the causal role of 'UE activity' is not established. Moreover, the specific attribution to MPI and color reconnection is not tested by any generator variation; it is an unsupported hypothesis. The absence of a long-range component in the CD correlators does not help, since a multiplicity-driven ridge would also be charge-independent. The 'exclusively' claim for the highest R_T class also depends on the ZYAM procedure and hand-chosen bins, but the primary gap is the missing control for multiplicity.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper uses PYTHIA 8 with the Monash 2013 tune to study two-particle number (R2) and transverse-momentum (P2) correlation functions in pp collisions at sqrt(s)=13 TeV, classifying events by the relative transverse activity RT. The central observation is that a long-range near-side component appears in the charge-independent correlator R2^CI only for the highest RT class (2.5 < RT <= 5.0), while the charge-dependent correlators show no such component. The authors interpret this as evidence that enhanced underlying-event activity, driven by multiple partonic interactions and color reconnection, can generate collectivity-like long-range correlations without hydrodynamic evolution, and they propose RT as a differential event classifier for non-hydrodynamic baselines.","tokens_in":14007,"tokens_out":3301,"duration_ms":34546,"significance":"If the interpretation holds, the paper would provide a useful PYTHIA baseline for LHC small-system studies and would sharpen the debate on whether ridge-like signals in pp collisions can arise from soft-QCD mechanisms. The differential comparison of CI and CD correlators as a function of RT is a sensible approach, and the observation of a long-range near-side component in R2^CI for high-RT events is qualitatively visible in the projections of Fig. 6. However, the strength of the claim currently exceeds what the analysis establishes: the RT classes are strongly correlated with total multiplicity, the statistical and systematic uncertainties are not quantified, and the attribution to MPI and color reconnection is not tested by any model variation. The paper is therefore a promising baseline study, but the central causal interpretation needs additional controls and a more cautious formulation.","major_comments":[{"comment":"The central claim is that enhanced UE activity, not high multiplicity per se, produces the long-range component. However, Fig. 3 shows that N_ch increases monotonically with RT, so the highest-RT class is also the highest-multiplicity class. The paper explicitly acknowledges this correlation but does not provide a multiplicity-matched control. Without such a control, the ridge observed in the 2.5 < RT <= 5.0 class could be a high-multiplicity selection effect rather than a UE-specific effect. Please add an N_ch-matched comparison (e.g., bin events at fixed RT in sub-bins of N_ch, or compare equal-N_ch samples with different RT) or clearly restrict the claim to 'RT-selected high-activity events' rather than 'UE activity causes the ridge.'","section":"§II, Fig. 3; §IV.A, Fig. 6"},{"comment":"No statistical uncertainties are shown in any of the correlation plots or RMS-width figures. With about 5e6 selected events, statistical errors can and should be quantified; they are essential for supporting the 'finite long-range component emerges' statement and the 'no corresponding structure' conclusion. In addition, the ZYAM subtraction is not fully specified: the polynomial order and the fit range in the near-side region are not given. Since the 'exclusively' claim and the RMS widths depend directly on this subtraction, please specify the procedure and test the sensitivity of the long-range signal to the ZYAM choices.","section":"§IV.A, Fig. 6; §IV.C, Figs. 9–12"},{"comment":"The abstract and summary assert that the long-range component is 'driven by multiple partonic interactions and color reconnection,' but no generator-level test is performed. This attribution is an unsupported hypothesis: PYTHIA 8 contains many coupled mechanisms, and the observed pattern could also reflect other features of the Monash tune (e.g., string shoving, hadronization parameters). Please either run controlled variations (e.g., switching color reconnection on/off, varying the MPI regularization scale) or explicitly state that the MPI/CR interpretation is a plausible but untested mechanism.","section":"§IV.A and §V (Summary)"},{"comment":"The statement that the long-range near-side component appears 'exclusively' for the highest RT class is too strong given the analysis shown. The lowest-RT class also exhibits sizable correlations at |Delta eta| > 1.2, which are dismissed as hard-scattering/jet contributions without a quantitative separation. A quantitative definition of the long-range yield, with its statistical significance, is needed before claiming exclusivity. Also, the adjacent intermediate class (1.5 < RT <= 2.5) shows a small but nonzero residual in the ZYAM-corrected projection; the distinction between 'substantially reduced' and 'absent' should be backed by uncertainties.","section":"§IV.A, Fig. 6 (left)"}],"minor_comments":[{"comment":"Typo: 'activi y' should be 'activity.'","section":"§V (Summary)"},{"comment":"The vertical-axis label appears malformed ('1/N_ev d^2 N_ch/(d eta d phi)'); the normalization and logarithmic scale should be clearly defined.","section":"Fig. 3"},{"comment":"The notation p_T,min and p_T,max is used but not defined explicitly; please state that these are the lower and upper bounds of the p_T acceptance.","section":"Eq. (5)"},{"comment":"The integration domain Omega for the average over eta_bar and phi_bar is not specified. Please define it (presumably the acceptance region |eta_i|<0.8 and the full azimuth).","section":"Eq. (6)"},{"comment":"Reference [10] is missing publication year/volume; several references have incomplete journal information (e.g., [7], [8], [9]). Please unify the bibliography style.","section":"References"}],"recommendation":"major_revision","confidential_remarks":"The paper is a purely simulation-based study and does not provide analysis code or Pythia configuration files; making these available would strengthen reproducibility. The main technical gap is the absence of a multiplicity-matched control, which is fixable within the scope of a revision. The authors should also consider citing recent ALICE pp measurements of R2 and P2 more fully, since Ref. [25] appears closely related."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Short version: this is a useful and clearly written PYTHIA baseline study, but the causal claim at the center—that enhanced UE activity (specifically MPI+CR) generates the ridge—is not actually tested. The new piece is the combination of the relative transverse activity classifier with R2/P2 charge-independent/charge-dependent decomposition, and the result that in the highest R_T bin R2^CI develops a long-range near-side component while P2^CI and the CD correlators don't. That's a concrete, falsifiable pattern and a plausible non-hydrodynamic reference for small-system collectivity studies.\n\nWhat the paper does well: the observable definitions are standard and clearly laid out, the event selection is reasonable (leading pT 5–40 GeV, using the transverse plateau), and the qualitative difference between CI and CD is demonstrated with projections and full 2D plots. It also cites the relevant PYTHIA ridge literature (Ortiz et al., Martin–Skands–Farrington) and the recent ALICE R2/P2 measurement.\n\nThe soft spots are real and mostly cluster around interpretation. First, R_T is correlated with total multiplicity (their own Fig. 3 shows it), so the highest R_T bin is also the highest-multiplicity bin. Without a multiplicity-matched control or an N_ch-binned comparison, the long-range component could be a high-multiplicity selection effect rather than something specific to transverse-region UE activity. Second, the attribution to MPI and color reconnection is asserted, not tested; no runs with CR or MPI disabled. That's the load-bearing step from \"PYTHIA produces a ridge at high R_T\" to \"UE activity drives the ridge.\" Third, the \"exclusively\" claim depends on hand-chosen R_T bins and a ZYAM procedure that is not fully specified (polynomial order, fit range). Statistical uncertainties are absent from all figures. With 5e8 events the errors are presumably small, but for a claim of absence in one bin and presence in another, you need them.\n\nNone of this kills the paper. The central observation is model output, not a derivation, and the qualitative pattern is likely robust. The fixes are straightforward: add binned or matched controls for N_ch, run with CR/MPI toggled, provide the ZYAM details and error bars, and soften \"exclusively\" to \"predominantly.\" If those land, this becomes a solid reference point.\n\nWho it's for: people working on small-system collectivity, and experimentalists who want a PYTHIA-based baseline for R2/P2 in pp. I'd send it to a serious referee; I'd cite it as a PYTHIA baseline if I were working in that area, with the caveat about the missing controls.","headline":"Useful PYTHIA baseline for R2/P2 with RT classification, but the causal claim about MPI/CR is untested and the R_T/N_ch correlation is not controlled.","tokens_in":14577,"tokens_out":2020,"would_cite":true,"duration_ms":18775,"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":"In pp collisions simulated with PYTHIA 8, selecting events with high underlying-event activity (2.5 < R_T ≤ 5.0) produces a long-range near-side ridge in the charge-independent two-particle number correlator R₂^CI, without any hydrodynamic","keywords":["underlying event","relative transverse activity","two-particle correlations","long-range ridge","collectivity","PYTHIA 8","color reconnection","multiple partonic interactions"],"falsifier":"Running the identical analysis with PYTHIA 8 configurations in which multiple partonic interactions and/or color reconnection are switched off: if the long-range near-side component in R₂^CI for 2.5 < R_T ≤ 5.0 persists, the attribution to MPI/CR is wrong; if it disappears, the claim is supported. Alternatively, repeating the R_T binning at fixed N_ch would falsify the UE-specific interpretation if the ridge vanishes when multiplicity is held constant.","tokens_in":13593,"feed_emoji":"⚛️","tokens_out":4529,"duration_ms":38050,"temperature":0.7,"pith_summary":"The paper uses PYTHIA 8 to study two-particle number and transverse-momentum correlations in 13 TeV proton-proton collisions, sorting events by relative transverse activity R_T, a measure of underlying-event (UE) activity. It finds that the charge-independent number correlator R₂^CI develops a finite long-range near-side component — a ridge — only in the most UE-dominated class (2.5 < R_T ≤ 5.0). No such long-range structure appears in the charge-dependent correlators, and the momentum correlator P₂^CI remains jet-like. The authors argue that enhanced UE activity, driven by multiple partonic interactions and color reconnection, can generate collectivity-like correlations without hydrodynamic expansion. The point of the study is to supply a non-hydrodynamic PYTHIA baseline for interpreting ridge-like signals in small-system LHC measurements.","feed_headline":"High underlying-activity pp events show a ridge in PYTHIA 8","feed_subtitle":"Selecting events by R_T exposes a charge-independent near-side ridge, a non-hydrodynamic baseline for LHC data.","key_machinery":"The relative transverse activity R_T, defined as the charged-particle density in the azimuthal transverse region (60° < |Δφ| < 120° around the leading particle) normalized by its event-ensemble average, is the classifier that separates events by underlying-event activity. The observables are the two-particle number correlator R₂ and transverse-momentum correlator P₂, decomposed into charge-independent and charge-dependent combinations. R₂ weights all pairs equally and is sensitive to soft UE contributions; P₂ weights pairs by Δp_T Δp_T and is sensitive to harder, more collimated particles. The contrast between CI and CD correlators, together with R_T binning, is what isolates the long-range,","core_discovery":"The central claim is that a finite long-range near-side component emerges in the charge-independent correlator R₂^CI for UE-dominated events classified by 2.5 < R_T ≤ 5.0, while no corresponding long-range structure is observed in the charge-dependent correlators R₂^CD and P₂^CD. The authors interpret this as evidence that enhanced underlying-event activity — mainly multiple partonic interactions and color reconnection in PYTHIA 8 — can generate collectivity-like long-range correlations without hydrodynamic evolution. As R_T increases, the CI near-side peak narrows in Δφ, while CD peaks stay localized with mild broadening; away-side structures weaken. The transverse-momentum correlator P₂^CI","pith_inferences":["A testable extension would be to run the same analysis with PYTHIA configurations in which multiple partonic interactions or color reconnection are disabled; if the high-R_T ridge disappears, the causal attribution would be confirmed.","Because N_ch rises monotonically with R_T in the shown multiplicity distributions, the ridge seen in the highest R_T class could partly be a high-multiplicity selection effect; a multiplicity-matched control across R_T bins would settle this.","The same R_T classification could be applied to experimental pp data and to other generators (e.g., with and without string shoving or rope hadronization) to see how model-dependent the ridge is.","The narrowing of the CI near-side peak in Δφ as R_T increases suggests that high-UE events are more azimuthally collimated even in the soft sector; this could be compared with color-reconnection-sensitive observables."],"forward_implications":["If correct, ridge-like signals in high-multiplicity pp and p–Pb collisions can be reproduced by microscopic QCD mechanisms (MPI plus color reconnection) without invoking a hydrodynamic medium.","R_T becomes a practical differential event classifier: UE-dominated events (2.5 < R_T ≤ 5.0) can be selected to isolate soft-QCD-driven long-range correlations.","The near-side Δφ narrowing of R₂^CI with increasing R_T predicts an observable trend in experimental two-particle correlations across event-activity classes.","The absence of long-range structure in CD correlators predicts that any measured ridge in pp collisions should be charge-independent, with balance functions remaining short-range.","The different behavior of R₂ versus P₂ provides a way to separate soft UE effects from jet-dominated correlations in data."],"fun_headline_variants":["High R_T pp events show ridge in PYTHIA 8, no hydro","R_T-driven ridge in pp: PYTHIA 8, no hydro needed","Collectivity-like ridge appears only at high R_T in pp","Charge-independent ridge in pp requires high R_T, no hydro","PYTHIA 8 pp: high-activity events mimic collectivity"],"cache_read_input_tokens":2304,"weakest_assumption_plain":"The load-bearing premise is that the highest-R_T class isolates enhanced underlying-event activity as the cause of the ridge, even though that class also has the highest charged-particle multiplicity and the paper shows no multiplicity-matched control, so the long-range component could be a high-multiplicity selection effect rather than a UE-specific one.","fun_headline_variants_meta":{"raw":{"variants":["High R_T pp events show ridge in PYTHIA 8, no hydro","R_T-driven ridge in pp: PYTHIA 8, no hydro needed","Collectivity-like ridge appears only at high R_T in pp","Charge-independent ridge in pp requires high R_T, no hydro","PYTHIA 8 pp: high-activity events mimic collectivity"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000968,"raw_usage":{"total_tokens":3991,"prompt_tokens":818,"completion_tokens":3173,"prompt_tokens_details":{"cached_tokens":256},"prompt_cache_hit_tokens":256,"prompt_cache_miss_tokens":562,"completion_tokens_details":{"reasoning_tokens":3077}},"tokens_in":562,"tokens_out":3173,"duration_ms":18723,"temperature":1.0,"reasoning_tokens":3077,"cache_read_input_tokens":256,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-02T21:35:11.961205+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Running the identical analysis with PYTHIA 8 configurations in which multiple partonic interactions and/or color reconnection are switched off: if the long-range near-side component in R₂^CI for 2.5 < R_T ≤ 5.0 persists, the attribution to MPI/CR is wrong; if it disappears, the claim is supported. Alternatively, repeating the R_T binning at fixed N_ch would falsify the UE-specific interpretation if the ridge vanishes when multiplicity is held constant.","supporting_citations":[],"review_version":1}