{"id":"99de3540-a53d-48a0-a666-0e71f658613f","arxiv_id":"1908.11238","paper_version":1,"verdict":"CONDITIONAL","confidence":"HIGH","novelty_score":4.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":4,"one_line_summary":"New CTEQ-TEA global fits CT18 and CT18Z provide updated NNLO proton parton distributions from LHC and world data, with CT18Z including the ATLAS 7 TeV W/Z measurements.","lead":"Physicists fitted two new sets of proton structure functions, CT18 and CT18Z, to the latest LHC and older collider measurements. The two sets differ in whether they include ATLAS's 7 TeV W and Z data, a choice that affects many LHC predictions.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"K-factor systematics for NNLO jet/high-pT Z predictions are unquantified; the 0.5% error covers numerical integration only, so the fitted gluon may be biased.","rationale":"The reader's conditional verdict is appropriate. I focus on the K-factor route because the paper's headline NNLO claim, and therefore its usefulness as a standard reference, depends on it. The paper is transparent that NNLO predictions are obtained indirectly for jets and high-pT Z, and it explicitly adds a 0.5% uncorrelated error for numerical integration noise. Yet no systematic error is assigned to the NLO-grid x K-factor approximation itself. This asymmetry is a concrete gap: one known uncertainty is modeled, while another potentially larger one is not. The ATLAS jet decorrelation issue makes the gap more salient, since the paper notes that the fit quality remains suboptimal even after decorrelation. My proposed test is computationally demanding but decisive: a full NNLO refit would show whether the K-factor approximation changes the extracted gluon beyond its quoted uncertainty. This does not overturn the reader's conditional verdict; it sharpens the condition that must be met before CT18/CT18Z can be treated as a fully validated NNLO reference.","tokens_in":6650,"tokens_out":6269,"duration_ms":66845,"concrete_test":"Using the released CT18 grids and the same data selection, replace the NLO-APPLgrid x K-factor predictions for the ATLAS 7 TeV inclusive jet and ATLAS 8 TeV high-pT Z bins with full NNLO calculations computed with the fitted CT18 PDFs, and refit. If the gluon PDF at Q = 125 GeV shifts by more than the 90% Hessian band at x = 0.01-0.3, or if the per-bin chi2 changes by more than the quoted experimental uncertainties, the K-factor approximation is the limiting systematic and the reported PDF uncertainties must be inflated.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The paper's NNLO claim for the new LHC jet and high-pT Z data rests on NLO APPLgrid tables multiplied by NNLO/NLO K-factors, as stated in the theory description. A 0.5% uncorrelated error is added to cover numerical uncertainties in the Monte Carlo integration of NNLO cross sections, but no corresponding systematic uncertainty is assigned to the K-factor approximation itself. For inclusive jets, K-factors are known to have nontrivial pT and rapidity dependence and to depend on the input PDF set used to compute them; applying a bin-wise multiplicative correction computed at a fixed reference PDF can distort the fitted gluon if the true NNLO shape differs from NLO x K-factor. The issue is compounded by the ATLAS 7 TeV jet data: the paper states that even after the ATLAS-recommended decorrelations, the resultant chi2 is not optimal and the constraints are less effective. If K-factor shape errors exceed the experimental precision of the jet and high-pT Z bins, the CT18/CT18Z central values and Hessian uncertainty bands are overconfident. This is the most load-bearing uncertainty because those data are among the principal new LHC constraints driving the difference from CT14HERA2.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"This proceedings paper reports the CTEQ-TEA global QCD analysis CT18 and its variant CT18Z, which determine nucleon parton distribution functions at NNLO in the Hessian method. CT18 includes a broad set of new LHC data (jets, W/Z, Drell-Yan, high-pT Z, top-quark pair production) while CT18Z additionally includes ATLAS 7 TeV W and Z rapidity data and differs in the treatment of CDHSW data, the charm quark mass, and the DIS saturation scale. The paper presents global chi2/Npt values of 1.17 (CT18) and 1.19 (CT18Z), plots of the resulting PDF uncertainties relative to CT14HERA2, Lagrange Multiplier scans for the gluon and strangeness ratio, and comparisons of LHC PDF luminosities with MMHT2014 and NNPDF3.1.","tokens_in":6889,"tokens_out":3149,"duration_ms":31198,"significance":"If the CT18/CT18Z sets are reliable, they constitute an updated standard reference for LHC phenomenology, since they incorporate several high-precision LHC data sets into a global Hessian NNLO analysis. The paper is commendably explicit about the data-selection choices: it states that ATLAS 7 TeV W/Z data are excluded from CT18 and included in CT18Z, and it reports the resulting chi2/Npt values. The Lagrange Multiplier scans are a useful diagnostic of which data constrain the gluon and strangeness PDFs. As a proceedings contribution, however, the paper leaves several load-bearing methodological details to the companion literature, and therefore the quantitative claims should be treated as preliminary pending the full CT18 publication.","major_comments":[{"comment":"The NNLO treatment of the inclusive jet and high-pT Z data relies on NLO APPLgrid tables multiplied by NNLO/NLO K-factors, with only a 0.5% uncorrelated error assigned for numerical integration. The paper does not quantify the uncertainty of the K-factor approximation itself, including its dependence on pT, rapidity, and the input PDF set used to compute the K-factors. Because the ATLAS 7 TeV jet data are retained (albeit with decorrelations and the acknowledged non-optimal chi2) and are among the principal new constraints on the gluon, a K-factor shape error could bias the fitted gluon and the resulting NNLO PDFs. Please provide an estimate of this systematic, or justify with a concrete test why it is negligible relative to the experimental precision of the fitted jet and high-pT Z data.","section":"Section 1 (Theoretical predictions)"},{"comment":"The 90% C.L. PDF error bands are a central deliverable of the paper, but the Hessian tolerance criterion used to define the error ensembles is not stated, nor is the functional form of the new 'flexible' parametrization specified beyond the claim that it is the same as CT14HERA2 for u, d, ubar, and dbar. Without the tolerance and the parametrization form, the quoted uncertainty bands cannot be reproduced or compared with other groups' sets. Please state the tolerance criterion and the parametrization choices, or explicitly refer to the full CT18 paper where these are defined.","section":"Section 1 and Figures 1-3, 6 (Hessian uncertainties)"},{"comment":"The paper states that CT18 assumes exact SU(3) symmetry of the sea quark PDFs so that (s+sbar)/(ubar+dbar) approaches 1 as x tends to 0, and that this ratio at x<1e-3 is determined entirely by the parametrization form. This is a model assumption imposed on the fit, not a data-driven result, and it directly affects the reported increase in the strangeness PDF at x<0.03. The discussion should clearly flag that the small-x strangeness ratio is an input assumption, and should indicate how the uncertainty bands in Fig. 3 account for this choice.","section":"Section 2 (Strange PDF and exact SU(3) assumption)"}],"minor_comments":[{"comment":"The notation 'CT18(Z)' and 'chi2/Npt = 1.17(1.19)' is ambiguous; state explicitly which number corresponds to CT18 and which to CT18Z.","section":"Abstract and Section 1"},{"comment":"The sentence 'In some kinematic regions, there are few constraints... Lagrange Multiplier constraints are then applied' would benefit from a reference to the exact Lagrange Multiplier procedure or a definition of the applied constraints, so the reader can distinguish them from the uncertainty scans shown in Figs. 4 and 5.","section":"Section 1"},{"comment":"The statement that the parametrization form of u, d, ubar, and dbar in CT18 is the same as in CT14HERA2, while 'new flexible PDF parametrizations have been tested for CT18', is confusing and should be reconciled.","section":"Section 2"},{"comment":"The caption says the error bands are normalized to the 'respective central CT14HERA2 NNLO PDFs'; since the bands are displayed for CT18 and CT18Z relative to a fixed CT14HERA2 reference, please rephrase to avoid implying that each band is normalized to its own central value.","section":"Figure 1 caption"}],"recommendation":"major_revision","confidential_remarks":"This is a proceedings-style contribution, and several missing details are likely deferred to the full CT18 papers. However, the unquantified K-factor approximation for the NNLO jet and high-pT Z predictions is a genuine load-bearing concern, since the paper explicitly calls out the non-optimal chi2 of the ATLAS 7 TeV jet data after decorrelations. The authors should be asked to either provide a quantification of the K-factor systematic or explicitly state that the proceedings results are provisional and refer to the forthcoming full analysis for the systematic assessment. The Hessian tolerance and parametrization details should likewise be stated or referenced precisely."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"CT18 and CT18Z are exactly what the title says: the CTEQ global analysis updated with a broad set of LHC data, split into two PDF families according to whether the ATLAS 7 TeV W/Z rapidity measurements are fitted. That split is the genuinely new piece, and it is handled honestly — the authors state plainly that the two data sets are in tension and declare the accompanying variations (CDHSW removed, charm mass moved from 1.3 to 1.4 GeV, saturation scale used in DIS). The final chi2/Npt around 1.17–1.19 is reasonable for a fit this size, the Lagrange multiplier scans for the gluon and for the strangeness ratio show where the constraints actually come from, and the luminosity comparison with MMHT and NNPDF is a useful sanity check. They also admit that the high-x d/u behavior reflects the parametrization form rather than data, which is the right kind of transparency.\n\nThe soft spots are mostly a consequence of this being a short proceedings. Parametrization form, Hessian tolerance, and the correlated-systematics model beyond the ATLAS decorrelations are not specified; users have to wait for the companion paper and the released grids. That is a real gap for output meant to be used by others.\n\nThe load-bearing issue is the NNLO treatment of the jet and high-pT Z data. These enter through NLO APPLgrid tables multiplied by NNLO/NLO K-factors, and the 0.5% uncorrelated error is explicitly for numerical integration of the NNLO cross sections, not for the K-factor approximation itself. Inclusive-jet K-factors have genuine pT and rapidity dependence, and they depend on the input PDF set used to compute them; a bin-wise multiplicative correction computed at a fixed reference PDF can distort the fitted gluon. The paper itself concedes that the ATLAS 7 TeV jet chi2 is not optimal even after the recommended decorrelations. The stress-test concern lands: it is an unquantified systematic in some of the principal new data sets that drive the difference from CT14HERA2. I would not call it a demonstrated flaw — the companion paper may quantify the sensitivity — but it belongs on the referee's checklist.\n\nOne smaller point: the abstract promises predictions of standard candles, yet several of those processes are inside the fitted ensemble; the paper never identifies which predictions are genuinely outside the fit.\n\nWho benefits: phenomenologists who need a current reference PDF set for LHC predictions, and other groups comparing global-fit methodology. The proceedings alone is thin; the value is in the CT18 sets and the companion paper. This deserves serious refereeing in that context — the work is important enough that the missing details should be chased down, not waved away.","headline":"CT18/CT18Z is a workmanlike update of the CTEQ program with new LHC data, transparently split over the ATLAS 7 TeV W/Z tension; the proceedings is thin, and the K-factor systematics for jets are the real open question.","tokens_in":7485,"tokens_out":3389,"would_cite":true,"duration_ms":31150,"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 new CTEQ global analysis fits nucleon parton distributions at NNLO to LHC precision data and issues two families, CT18 and CT18Z, that differ in whether the challenging ATLAS 7 TeV W and Z data are included.","keywords":["parton distribution functions","global QCD analysis","NNLO","Hessian method","LHC data","CT18","CT18Z","strangeness ratio"],"falsifier":"Recompute the inclusive jet cross sections entering the fit directly at NNLO with exact grids (no $K$-factor approximation), refit the CT18 ensemble, and compare the gluon PDF at $x=0.01$ and $x=0.3$ at $Q=125$ GeV; if the central gluon moves by more than the quoted 90% C.L. band, the $K$-factor pipeline is the source. A second, decisive check would be a new ATLAS $W/Z$ rapidity measurement whose systematics allow it to join a single global fit without degrading $\\chi^2$, which would erase the need for the two-family split.","tokens_in":6434,"feed_emoji":"⚛️","tokens_out":7635,"duration_ms":67808,"temperature":0.7,"pith_summary":"CT18 is a new global QCD analysis that determines the nucleon parton distribution functions (PDFs) at next-to-next-to-leading order using the Hessian method, fitting 30 LHC candidate data sets together with legacy deep-inelastic, Tevatron, and HERA data. Because the ATLAS 7 TeV $W$ and $Z$ rapidity measurements cannot be accommodated in the same fit as the other data, a second family, CT18Z, is produced that includes those data while removing CDHSW, raising the charm pole mass from $1.3$ to $1.4$ GeV, and using a saturation scale for DIS processes. The final fits give $\\chi^2/N_{\\rm pt}=1.17$ for CT18 (3681 points) and $1.19$ for CT18Z (3493 points). A reader should care because these sets are offered as an updated reference for LHC standard-candle predictions, and the comparison between the two families reveals which data and modelling choices still matter.","feed_headline":"Two new CTEQ PDF sets absorb LHC precision data at NNLO","feed_subtitle":"CT18 and CT18Z update the parton picture of the proton, with and without the ATLAS 7 TeV W/Z data.","key_machinery":"The mechanism that carries the analysis is the NNLO theory pipeline built from fast interpolation tables: NLO APPLgrid grids multiplied by NNLO/NLO $K$-factors for jet and vector-boson production, and fastNLO grids for top-pair observables. The Hessian method—approximating the parameter dependence of $\\chi^2$ by a quadratic form and diagonalising it into error eigenvectors—turns the data ensemble into the 90% confidence-level PDF bands. Lagrange Multiplier scans probe non-quadratic constraints on selected PDF features, and a $0.5\\%$ uncorrelated error absorbs residual Monte Carlo integration uncertainties of the NNLO cross sections.","core_discovery":"The paper claims that a global NNLO fit can absorb a broad set of high-precision LHC measurements—inclusive jets, $W$, $Z$, high-$p_T$ $Z$, Drell-Yan, and $t\\bar t$ production—and that the resulting CT18 PDFs sharpen the constraints on the gluon at $x \\sim 0.01$–$0.3$, reduce the $d/u$ ratio at $x > 0.5$, and set the strange-to-nonstrange sea ratio toward SU(3)-symmetric behaviour at small $x$. The ATLAS 7 TeV $W/Z$ data are the one major set that cannot be fitted together with the others; the CT18Z variant includes them at the price of three associated changes, and the two fits end with nearly equal global $\\chi^2$ per point. The paper presents the two families as complementary, with the differences between them quantifying the sensitivity of PDFs to that data set and to the treatment of charm and DIS scales.","pith_inferences":["If a future theory improvement—for example genuine NNLO electroweak corrections or a more physical charm-mass treatment—removes the ATLAS 7 TeV $W/Z$ tension, the two-family structure would likely collapse into one fit; the CT18/CT18Z split is an existence proof that current NNLO QCD does not yet reconcile all high-precision LHC data.","A testable extension is to compare CT18 and CT18Z predictions for the 13 TeV lepton charge asymmetry of $W$ production; because the sets differ most in the $\\bar d/\\bar u$ and strangeness ratios, high-rapidity asymmetry data at 13 TeV could discriminate between them.","The quoted $\\chi^2/N_{\\rm pt}\\approx 1.17$ includes a $0.5\\%$ uncorrelated numerical error; if faster exact NNLO calculations reduce that tolerance, the same data would yield narrower PDF uncertainty bands, so the reported bands may be read as mildly conservative."],"forward_implications":["If CT18 and CT18Z become the reference PDFs, LHC predictions for $W$, $Z$, Higgs, and top-pair cross sections will be quoted with the error bands from these sets, and the two families give an immediate way to estimate the PDF shift caused by the ATLAS 7 TeV $W/Z$ data.","The reduced $d/u$ ratio at $x>0.5$ in CT18 will lower predictions for processes sensitive to valence quarks at high momentum fraction, such as forward $W$ production and certain $t\\bar t$ asymmetries.","The SU(3)-symmetric strangeness assumption at $x\\to 0$, combined with data constraints near $x=0.023$, gives a definite $R_s$ profile that enters predictions of neutrino-nucleon and $W$+charm processes.","The near-equal $\\chi^2$ of the two fits means that analyses comparing with ATLAS 7 TeV $W/Z$ data should state explicitly which family they use, since CT18 and CT18Z differ in sea-quark flavour ratios."],"supporting_citations":[{"why":"Supplies the predecessor CT14HERA2 PDFs against which CT18 uncertainties and shifts are normalized.","marker":"[1]"},{"why":"ATLAS 7 TeV inclusive jet production data supply a main new constraint on the gluon and require the systematic decorrelation treatment.","marker":"[2]"},{"why":"The NNLO/NLO K-factor calculation for jet production is the basis for promoting NLO grid predictions to NNLO accuracy in the fit.","marker":"[3]"},{"why":"ATLAS 7 TeV W and Z rapidity distributions are the data set whose inclusion defines the separate CT18Z family.","marker":"[4]"},{"why":"NLO APPLgrid fast interpolation tables provide the grids that are multiplied by K-factors for jet and vector-boson processes.","marker":"[5]"},{"why":"The NNLO top-pair cross-section grids supply the direct NNLO theory for the ttbar observables.","marker":"[6]"},{"why":"fastNLO supplies the numerical interpolation machinery used for the top-pair NNLO grids.","marker":"[7]"},{"why":"CMS 7 TeV jet data provide the complementary gluon constraint used alongside the ATLAS jet measurements.","marker":"[8]"},{"why":"ATLAS 8 TeV high-pT Z data constrain quark-antiquark annihilation and enter the ensemble with the 0.5% numerical error allowance.","marker":"[10]"}],"fun_headline_variants":["Two new CTEQ PDFs: CT18 and CT18Z for LHC","CT18 and CT18Z: LHC data with and without ATLAS","CTEQ's latest: CT18 and CT18Z for precision LHC","Proton parton puzzle: CT18 vs CT18Z"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The analysis stands or falls on the assumptions that NNLO predictions for jets and high-$p_T$ $Z$ production are adequately represented by NLO fast grids multiplied by NNLO/NLO $K$-factors, and that the ATLAS-recommended systematic decorrelations correctly capture the correlated errors of the 7 TeV jet data; if either assumption is wrong, the extracted gluon and quark distributions shift.","fun_headline_variants_meta":{"raw":{"variants":["Two new CTEQ PDFs: CT18 and CT18Z for LHC","CT18 and CT18Z: LHC data with and without ATLAS","CTEQ's latest: CT18 and CT18Z for precision LHC","Proton parton puzzle: CT18 vs CT18Z"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000326,"raw_usage":{"total_tokens":1812,"prompt_tokens":919,"completion_tokens":893,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":535,"completion_tokens_details":{"reasoning_tokens":812}},"tokens_in":535,"tokens_out":893,"duration_ms":8431,"temperature":1.0,"reasoning_tokens":812,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-14T10:20:53.108138+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Recompute the inclusive jet cross sections entering the fit directly at NNLO with exact grids (no $K$-factor approximation), refit the CT18 ensemble, and compare the gluon PDF at $x=0.01$ and $x=0.3$ at $Q=125$ GeV; if the central gluon moves by more than the quoted 90% C.L. band, the $K$-factor pipeline is the source. A second, decisive check would be a new ATLAS $W/Z$ rapidity measurement whose systematics allow it to join a single global fit without degrading $\\chi^2$, which would erase the need for the two-family split.","supporting_citations":[],"review_version":1}