{"id":"14d4aa04-9e27-496f-9761-8b9371713188","arxiv_id":"2412.15658","paper_version":3,"verdict":"ACCEPT","confidence":"HIGH","novelty_score":6.0,"correctness_risk":"low","formal_verification":"none","parameter_count":0,"one_line_summary":"ATLAS reports positive elliptic flow v2 up to charged particle pT about 100 GeV and triangular flow v3 up to about 25 GeV in Pb+Pb collisions at 5.02 TeV, extending previous measurements to 400 GeV.","lead":"ATLAS measured how strongly charged particles from lead-lead collisions are emitted in an oval or triangular pattern, stretching the measurement to particle momenta up to 400 GeV. The results show a small but real elliptic anisotropy persists to around 100 GeV, a constraint on how jets lose energy in the quark-gluon plasma.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Residual dijet correlations via the FCal reference may inflate high-pT v2{SP}; a ZDC event-plane cross-check would settle it.","rationale":"The reader's weakest assumption identified residual back-to-back dijet correlations as the main threat to the interpretive claim. I agree and refine the mechanism: the FCal reference in the scalar-product method is itself a potential source of such dijet nonflow, since a high-pT charged particle is correlated with its parent jet, which can deposit energy in the FCal. The paper's data show trends (v2 flattening, v3 sign change in peripheral events, SP vs MPC differences) that are all consistent with this nonflow, and the authors explicitly acknowledge the ambiguity in Section 6.2. The central measurement itself is well-executed and the systematic uncertainties are comprehensive, so the data are not in question. However, the strongest claim as phrased in the conclusions—that positive v2 and v3 suggest energy loss is influenced by the event-by-event initial geometry—is not uniquely established until the nonflow contribution is quantitatively addressed. A ZDC event-plane cross-check, being insensitive to jet energy, would directly test whether the FCal-based v2 at high pT is contaminated. Since the paper already qualifies its interpretation and the reader's verdict remains appropriate for a measurement paper, I recommend no change to the verdict. The proposed test would provide a decisive resolution if performed in a follow-up analysis.","tokens_in":60059,"tokens_out":10352,"duration_ms":92893,"concrete_test":"Recompute v2{SP} for pT > 20 GeV using the event plane from the ZDC (spectator neutrons, |eta| > 8.3) rather than the FCal. The ZDC is insensitive to jet energy, so the resulting v2 is free from jet-induced nonflow. If the ZDC-based v2 is significantly smaller than the FCal-based v2 (or consistent with zero), the high-pT v2 is contaminated by jet nonflow, weakening the geometry interpretation. If it remains positive, the claim is supported. The ZDC resolution is lower, but the 2018 dataset should allow a test up to ~50 GeV with adequate statistical power.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central interpretive claim is that the positive v2{SP} up to 100 GeV and v3{SP} up to ~25 GeV reflect path-length-dependent parton energy loss, i.e., a response to the initial geometry. The scalar-product method (Sec. 4.1) uses FCal transverse energy as the reference flow vector. Although a pseudorapidity gap of 3.2 suppresses same-jet correlations, the away-side jet can deposit energy in the FCal, inducing a positive even-harmonic and negative odd-harmonic correlation that mimics flow. The paper's own observations are consistent with this: v2{SP} flattens above ~50 GeV in mid-central bins (Sec. 6.1), v3{SP} becomes negative in 50-60% peripheral events, and the SP vs MPC comparisons (Sec. 6.2, Fig. 14) show residual dijet contributions that differ between even and odd harmonics. Section 6.2 explicitly states that multiple effects, including dijets, could contribute to the observed trends. Therefore, the measured v2 at high pT may be contaminated by nonflow, and the interpretation that it is driven by the initial geometry is not uniquely supported. This does not challenge the data quality, but it undermines the strongest claim as stated in the conclusions.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"This paper reports measurements of the elliptic and triangular azimuthal anisotropy coefficients v2 and v3 for charged particles with transverse momentum from 1 to 400 GeV in Pb+Pb collisions at sqrt(s_NN) = 5.02 TeV, using 0.44 nb^-1 of ATLAS 2018 data. The measurement uses the scalar-product method with FCal reference flow vectors and the multiparticle cumulant method, including a pT-differential extension of the three-subevent Q-cumulant technique. The principal results are positive v2{SP} values up to pT of about 100 GeV and positive v3{SP} values up to about 25 GeV in mid-central collisions, with comparisons to CMS and ATLAS 2015 measurements and to ATLAS jet v_n measurements. The paper interprets the high-pT anisotropy as evidence that parton energy loss is influenced by the event-by-event initial geometry, while acknowledging residual nonflow effects, especially in peripheral events.","tokens_in":60295,"tokens_out":12263,"duration_ms":113721,"significance":"If the results hold, this analysis extends the transverse-momentum reach of charged-particle flow measurements by roughly a factor of four relative to earlier LHC results and provides a new differential three-subevent cumulant implementation that is useful for nonflow studies. The extraction is direct: v_n values are measured rather than fitted, the MC afterburner enters only in efficiency corrections, and the comparisons with independent CMS and ATLAS datasets provide useful cross-checks. The systematic uncertainty budgets in Figures 1 and 2 are detailed, and statistical uncertainties dominate at high pT as stated in Section 5. The main limitation is the interpretation of the high-pT anisotropy in terms of path-length-dependent energy loss, because residual dijet correlations can mimic even-harmonic signals; this limitation is explicitly discussed in Section 6.2. The specific worry that FCal reference contamination alone drives the high-pT v2 is not supported by the data, since positive v2 is also observed with the four-particle cumulant method, which does not use the FCal.","major_comments":[],"minor_comments":[{"comment":"The statements that v2{SP} is positive up to pT of about 100 GeV and v3{SP} positive up to about 25 GeV are not accompanied by an explicit significance threshold; please add a sentence specifying the statistical significance of the last positive pT bins, or qualify the wording as 'central values are positive' if the significance is marginal.","section":"Abstract and Section 6.1"},{"comment":"The abstract states that positive v3 values are observed up to about 25 GeV, but Section 6.1 reports negative v3{SP} for pT > 20 GeV in the 50-60% centrality interval; the abstract should mention this exception for clarity.","section":"Abstract and Section 6.1"},{"comment":"The final interpretive sentence of the conclusions should explicitly carry the caveat from Section 6.2 that multiple effects, including residual dijet contributions, could contribute to the observed high-pT trends in peripheral events.","section":"Section 7"},{"comment":"The captions of Figures 9 and 16 refer to the difference in v3{SP}, but panel (a) of both figures shows v2{SP}; the captions should read 'v_n{SP}' or be corrected to match the panels.","section":"Figures 9 and 16"},{"comment":"The phrase 'short-rage nonflow correlations' should be corrected to 'short-range nonflow correlations'.","section":"Section 6.2"},{"comment":"The word 'flucutations' should be corrected to 'fluctuations'.","section":"Section 7"},{"comment":"References [42] and [47] are identical (both are ATLAS vertex reconstruction performance notes); they should be merged or cross-referenced.","section":"References"},{"comment":"The DOI string '10.1103/d46f-yl4n' in the header appears to be a placeholder and should be replaced with the final DOI.","section":"Article header"}],"recommendation":"minor_revision","confidential_remarks":"This is a mature and careful analysis from ATLAS. The central measurement is sound, and the residual-nonflow caveat is already acknowledged in the manuscript. The requested changes are local wording and clarity improvements rather than a request for new analysis. The stress-test concern about FCal-induced dijet contamination does not change my assessment, because the positivity of v2 at high pT is also seen with the MPC method, which does not use FCal."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Short version: this is a solid, incremental but genuinely useful measurement paper. It extends the pT reach of charged-particle v_n from ~100 GeV to 400 GeV, and it introduces the first pT-differential three-subevent v_n{4} results. The analysis follows established ATLAS methodology and the systematics are documented in detail. The data will be a reference point for jet-quenching models. I agree with the ACCEPT verdict.\n\nThe new things: the three-subevent cumulant implementation for differential v_2 and v_3, the comparison of SP and MPC methods in peripheral bins, and the explicit study of event-combination procedures. These are careful, and the paper earns its claims.\n\nThe soft spot is the one the stress-test note flags, but it is already in the paper. The FCal reference vector can pick up away-side jet energy, which mimics flow: positive for even harmonics, negative for odd. The authors say this themselves in Secs. 6.1 and 6.2, and they show the peripheral v3{SP} going negative, which is the signature. So the claim that positive v2 up to 100 GeV reflects geometry-driven energy loss is not uniquely established. That said, the authors do not overclaim—they say 'suggest', and they flag the multiple possible contributions. A quantitative nonflow estimate would strengthen the paper, but its absence is not a fatal flaw; the measurement itself is the product.\n\nA ZDC event-plane cross-check is a reasonable idea but not a simple one: ZDC resolution is poor and the signal is small. I would have liked a template-fitting or a PYTHIA-embedded dijet study instead. But this is a minor quibble.\n\nBottom line: this paper will be used by jet-quenching modelers, and it deserves careful reading. If I were referee, I'd accept with comments asking for a quantitative dijet-nonflow estimate. I'd certainly cite it. Bring it to reading group if you want a good discussion on nonflow at high pT.","headline":"Solid, careful extension of the high-pT flow program; the nonflow caveats are real but already transparently acknowledged.","tokens_in":60816,"tokens_out":4668,"would_cite":true,"duration_ms":36415,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"In mid-central lead-lead collisions at 5.02 TeV, elliptic anisotropy of charged particles remains positive up to 100 GeV of transverse momentum, indicating that parton energy loss in the quark-gluon plasma depends on the event-by-event…","keywords":["azimuthal anisotropy","elliptic flow","triangular flow","quark-gluon plasma","jet quenching","parton energy loss","scalar product method","multiparticle cumulants"],"falsifier":"A measurement of $v_2$ for charged particles above 20 GeV using a method that removes back-to-back dijet contributions entirely (for example, four-subevent cumulants with all-particle pseudorapidity gaps, or a jet-vetoed event sample) that drives $v_2$ to zero or negative in mid-central collisions would contradict the claim that the positive high-$p_{\\text{T}}$ $v_2$ reflects path-length-dependent energy loss in the quark-gluon plasma.","tokens_in":59854,"feed_emoji":"🌀","tokens_out":6641,"duration_ms":52444,"temperature":0.7,"pith_summary":"This paper measures how strongly very high-momentum charged particles are aligned with the shape of the quark-gluon plasma created in lead-lead collisions at 5.02 TeV. Using the ATLAS 2018 dataset, it finds that elliptic anisotropy $v_2$ stays positive up to a charged-particle transverse momentum of 100 GeV in mid-central collisions, at about 1\\,--\\,2%, and triangular anisotropy $v_3$ stays positive up to about 25 GeV. Since particles above roughly 10 GeV come mostly from jet fragmentation, these persistent harmonics indicate that the energy loss of hard-scattered partons is influenced by the event-by-event geometry of the plasma. The paper extends the three-subevent cumulant technique to differential $v_2$ and $v_3$ and uses method comparisons to separate genuine geometry-driven flow from residual jet correlations.","feed_headline":"Elliptic anisotropy stays positive up to 100 GeV in Pb+Pb collisions","feed_subtitle":"New ATLAS data link the shape of the quark-gluon plasma to jet energy loss up to 100 GeV.","key_machinery":"The central objects are the Fourier coefficients $v_n$ of the azimuthal distribution of charged particles, estimated by two correlation methods: the scalar-product method ($v_n\\{SP\\}$), which uses forward-calorimeter flow vectors with a pseudorapidity gap, and four-particle cumulants ($v_n\\{4\\}$) computed with $Q$-cumulants, including a three-subevent variant that forces the correlated particles into separated pseudorapidity intervals. These estimators separate the long-range, geometry-driven anisotropy from short-range nonflow, and their cross-comparison is what allows the paper to attribute the high-$p_{\\text{T}}$ signal to path-length-dependent energy loss.","core_discovery":"At $\\sqrt{s_{_{\\text{NN}}}} = 5.02$ TeV, the ATLAS measurement shows that in mid-central lead-lead collisions the elliptic flow coefficient $v_2\\{SP\\}$ measured for charged particles with the scalar-product method remains positive up to a transverse momentum of 100 GeV, with a magnitude of about 1\\,--\\,2%, while triangular flow $v_3\\{SP\\}$ remains positive up to about 25 GeV. Because charged particles with $p_{\\text{T}}$ above roughly 10 GeV are produced mainly by jet fragmentation, the persistence of positive harmonics at such high $p_{\\text{T}}$ indicates that hard partons lose energy in a way that depends on the amount of quark-gluon plasma they traverse, i.e., on the event-by-event initial geometry. The paper also develops and applies a $p_{\\text{T}}$-differential three-subevent cumulant technique that suppresses short-range nonflow correlations, and uses comparisons among analysis strategies to show that residual back-to-back dijet correlations contribute positively to $v_2$ and negatively to $v_3$, especially in peripheral collisions.","pith_inferences":["Because back-to-back dijets push $v_2$ up and $v_3$ down, a joint fit to both harmonics at $p_{\\text{T}} > 20$ GeV could quantify and subtract this contamination, leaving a cleaner geometric signal for theory comparisons.","Tagging events with an isolated photon or $Z$ boson, where the hard parton direction is known, would provide a direct test of whether the positive $v_2$ at high $p_{\\text{T}}$ comes from path-length-dependent energy loss rather than from dijet kinematics.","The methodology of subevent cumulants in narrow centrality intervals is likely to become the default for future flow measurements at high $p_{\\text{T}}$, since it separates fluctuation sources without requiring large pseudorapidity coverage."],"forward_implications":["Positive $v_2\\{SP\\}$ at 1\\,--\\,2% up to 100 GeV and $v_3\\{SP\\}$ up to about 25 GeV extend the known high-$p_{\\text{T}}$ reach of flow harmonics and sharpen constraints on path-length-dependent jet quenching.","The three-subevent $Q$-cumulant method, now applied differentially in $p_{\\text{T}}$, suppresses short-range nonflow while still revealing residual back-to-back dijet contributions that push $v_2$ and $v_3$ in opposite directions in peripheral events.","Computing cumulants in narrow centrality bins before combining shows that $v_2\\{4\\}$ is primarily sensitive to nonflow fluctuations, while $v_3\\{4\\}$ is more sensitive to fluctuations of the initial triangular geometry.","Agreement with earlier ATLAS and CMS measurements validates the new partial-event-building dataset for high-$p_{\\text{T}}$ correlation studies."],"supporting_citations":[{"why":"CMS measurement of $v_n$ up to 100 GeV/c in PbPb collisions at 5.02 TeV; the main previous result this work extends and compares against.","marker":"[12]"},{"why":"ATLAS measurement of azimuthal anisotropies of jets; used to compare charged-particle $v_n$ with jet $v_n$ at high $p_{\\text{T}}$.","marker":"[13]"},{"why":"ATLAS 2015 measurement of charged-particle azimuthal anisotropy in 5.02 TeV Pb+Pb; provides the baseline for the 2018 dataset comparison.","marker":"[15]"},{"why":"Introduces the scalar-product method used to obtain $v_n\\{SP\\}$; defines the flow-vector estimator.","marker":"[17]"},{"why":"Generic framework for multiparticle azimuthal correlations with $Q$ cumulants; supplies the standard cumulant formalism used for $v_n\\{4\\}$.","marker":"[24]"},{"why":"Introduces three-subevent cumulants to suppress short-range nonflow; extended here to $p_{\\text{T}}$-differential $v_2\\{4\\}$ and $v_3\\{4\\}$.","marker":"[25]"},{"why":"Shows the importance of nonflow, especially back-to-back dijets, in multiparticle correlations; basis for interpreting opposite-sign $v_2$/$v_3$ behavior.","marker":"[26]"},{"why":"ATLAS study of anisotropic flow fluctuations; motivates the centrality event-combination procedures used to separate fluctuation sources.","marker":"[27]"},{"why":"ATLAS measurement of flow harmonics with multiparticle cumulants at 2.76 TeV; provides the earlier MPC comparison for $v_n\\{4\\}$.","marker":"[29]"}],"fun_headline_variants":["Flow signal seen up to 100 GeV in Pb+Pb collisions","ATLAS tracks jet energy loss via flow up to 100 GeV","Quark-gluon plasma shape imprints on jets at 100 GeV","Elliptic flow survives to record momentum in lead-lead"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The interpretation rests on the assumption that the measured high-$p_{\\text{T}}$ anisotropy is dominated by path-length-dependent parton energy loss in the quark-gluon plasma, rather than by residual back-to-back dijet correlations that survive the nonflow suppression.","fun_headline_variants_meta":{"raw":{"variants":["Flow signal seen up to 100 GeV in Pb+Pb collisions","ATLAS tracks jet energy loss via flow up to 100 GeV","Quark-gluon plasma shape imprints on jets at 100 GeV","Elliptic flow survives to record momentum in lead-lead"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000585,"raw_usage":{"total_tokens":2826,"prompt_tokens":1099,"completion_tokens":1727,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":715,"completion_tokens_details":{"reasoning_tokens":1652}},"tokens_in":715,"tokens_out":1727,"duration_ms":11666,"temperature":1.0,"reasoning_tokens":1652,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-11T11:12:11.184374+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"A measurement of $v_2$ for charged particles above 20 GeV using a method that removes back-to-back dijet contributions entirely (for example, four-subevent cumulants with all-particle pseudorapidity gaps, or a jet-vetoed event sample) that drives $v_2$ to zero or negative in mid-central collisions would contradict the claim that the positive high-$p_{\\text{T}}$ $v_2$ reflects path-length-dependent energy loss in the quark-gluon plasma.","supporting_citations":[],"review_version":1}