{"id":"1ff124c1-d51f-441b-914c-c062843106c7","arxiv_id":"1908.02223","paper_version":4,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":8,"one_line_summary":"The extracted F2^n/F2^p ratio saturates at 0.47 ± 0.04 as x approaches 1, supporting perturbative QCD and Dyson-Schwinger predictions over scalar diquark models.","lead":"The authors use deep inelastic scattering data from many nuclei to infer the structure of a free neutron, a quantity that cannot be measured directly because free neutrons are unstable. They find the neutron-to-proton quark momentum ratio saturates at 0.47 at high momentum, which favors one class of QCD symmetry-breaking models over another.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The 0.47 limit depends on funiv being both universal and deuterium-normalized; a closure test against a non-SRC Fermi-motion model is needed before the claim is secure.","rationale":"The reader's weakest assumption identifies the correct soft spot: the single universal modification function and SRC dominance at high x. My review sharpens the mechanism: because funiv is normalized to F2^d in Eq. 2, deuteron-specific high-x effects can be absorbed into the fitting function and then misapplied in Eq. 5, biasing F_n^2/F_p^2 where the paper claims saturation. The paper is otherwise internally consistent and acknowledges model sensitivity in the MARATHON R-function comparison, but the central numerical claim 0.47 ± 0.04 is conditional on this untested assumption. The natural test is a closure test with pseudo-data from an independent nuclear convolution model. Since the reader already returned CONDITIONAL, this review does not change the verdict.","tokens_in":10082,"tokens_out":6999,"duration_ms":81077,"concrete_test":"Perform a closure test: generate pseudo-data for F_A/F_d and F_p/F_d across the same nuclei and x_B range using a known input F_n^2/F_p^2 (e.g., 0.47 from pQCD and 0.25 from the scalar-diquark model) and a state-of-the-art convolution model that includes non-SRC high-x Fermi motion and off-shell effects (e.g., Kulagin-Petti). Then run the paper's HMCMC fit and Eq. 5 on these pseudo-data. If the recovered F_n^2/F_p^2 at x_B = 0.6–0.95 differs from the input by more than the quoted 0.04, the universality assumption cannot support the central limit.","verdict_should_be":"UNCHANGED","load_bearing_attack":"Eq. 2 defines funiv relative to F2^d, so the fit to F_A/F_d for A>2 cannot separate a truly universal bound-nucleon modification from a deuterium-specific high-x effect. Eq. 5 then uses that same funiv to correct deuterium and extract F_n^2/F_p^2. If F2^d has an excess high-x tail from binding, off-shell, or Fermi-motion that does not scale with the SRC abundance factor a2(A/d), funiv will absorb that deuteron-specific behavior and the extracted F_n^2/F_p^2 will be biased exactly in the x_B ≳ 0.6 region where the claim of saturation at 0.47 ± 0.04 is made. The paper explicitly assumes (Sec. II) that nucleon-motion effects above x_B ≈ 0.7 are dominated by short-range correlations and hence are proportional to SRC pair abundances; this is the load-bearing assumption. The quoted 0.04 uncertainty is the posterior width of the chosen parameterization, not the model uncertainty arising from this universality assumption. No closure test against a nuclear model with non-SRC Fermi-motion smearing is provided.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript presents a global analysis of nuclear deep inelastic scattering ratios F2^A/F2^d for nuclei from A = 3 to 208 within the short-range-correlation (SRC) universal-modification framework. Equation (1) models the nuclear structure function as a sum of free-nucleon contributions and contributions from nucleons in np-SRC pairs, leading to Eq. (2) with a nucleus-independent universal modification function f_univ. The authors perform a Bayesian Hamiltonian Markov Chain Monte Carlo fit of f_univ and F2^p/F2^d, and then use Eq. (5) to correct the deuteron data and extract F2^n/F2^p. The central result is that F2^n/F2^p becomes approximately constant for xB ≥ 0.6 and equals 0.47 ± 0.04 as xB → 1, in agreement with pQCD and Dyson-Schwinger predictions and in disagreement with the scalar diquark prediction. The paper also predicts F2^{3He}/F2^{3H}, the nuclear correction function R needed for the MARATHON extraction, and quantifies the model dependence of that extraction.","tokens_in":10430,"tokens_out":6741,"duration_ms":71872,"significance":"If the result holds, it would resolve a long-standing question about the neutron valence structure and discriminate between competing QCD-inspired symmetry-breaking mechanisms. The paper is valuable for combining the large A = 3 to 208 dataset into a single SRC-based correction scheme and for producing falsifiable predictions for the MARATHON and BONuS experiments. Strengths include the simultaneous Bayesian extraction of f_univ and F2^p/F2^d, explicit checks on low-W data and Q2 evolution, and an honest attempt to quantify the model uncertainty in the MARATHON correction function R. However, the central claim rests on the untested assumption that a single f_univ describes all nuclear modifications from xB = 0.08 to 0.95, including the Fermi-motion region xB > 0.7, and the quoted 0.04 uncertainty is a posterior interval rather than a model uncertainty. The paper would be substantially strengthened by a closure test against a conventional nuclear model with non-SRC Fermi-motion and binding corrections.","major_comments":[{"comment":"The assumption that a single universal modification function f_univ describes all nuclear modifications from xB = 0.08 to 0.95, including nucleon-motion effects at xB > 0.7, is explicitly stated in Sec. II ('This model assumes that both the EMC effect at 0.3 ≤ xB ≤ 0.7 and nucleon-motion effects ... are dominated by short-range correlations') and is load-bearing for the F2^n/F2^p extraction. No closure test is provided against a nuclear model in which high-x Fermi motion or binding effects are not proportional to the SRC abundance a2(A/d). Because Eq. (5) applies f_univ to correct deuterium, any failure of this assumption biases the extracted ratio precisely in the xB ≥ 0.6 region where the saturation claim is made. The authors should add an explicit test, for example replacing the SRC-scaled high-x term by a deuteron wave-function smearing model, and report how the extracted x → 1 limit changes. The quoted 0.04 posterior width does not include this model uncertainty.","section":"Sec. II, Eqs. (1)-(2)"},{"comment":"f_univ is defined in Eq. (3) relative to F2^d, so any deuteron-specific high-x contribution that does not scale with a2(A/d) is absorbed into f_univ and then reapplied to deuterium through Eq. (5). The fit to F2^A/F2^d for A > 2 cannot separate a universal bound-nucleon modification from a deuterium-specific excess. A quantitative estimate of the possible non-SRC high-x tail in F2^d, for instance from off-shell, binding, or Fermi-motion effects, and its propagation into F2^n/F2^p is needed before the 0.47 ± 0.04 limit can be considered robust. As it stands, both the central value and the uncertainty are conditional on the universality assumption, and the paper should explicitly acknowledge and quantify this dependence.","section":"Sec. II, Eqs. (3) and (5)"},{"comment":"The xB → 1 limit of 0.47 ± 0.04 is obtained by extrapolating the fitted four-parameter forms to xB = 1, while the fit constraints weaken toward xB ~ 0.95 and the low-W data (W < sqrt(2) GeV) are used for xB > 0.8. The robustness checks reported in the text, removing low-W data and evolving Q2, preserve the qualitative saturation but do not address the extrapolation uncertainty in the x → 1 limit. The manuscript should report the value and uncertainty obtained from the fit restricted to W ≥ sqrt(2) GeV at the highest x reached, and assess how much of the claimed limit is an extrapolation of the fitted functional form rather than a direct data constraint.","section":"Sec. II, Eq. (4) and Fig. 2"}],"minor_comments":[{"comment":"The equation as printed, F2^n/F2^p = 1 - f_univ / F2^p/F2^d - 1, is arithmetically inconsistent; it should be written as (1 - f_univ)/(F2^p/F2^d) - 1. Please correct the typography so the intended expression is unambiguous.","section":"Sec. II, Eq. (5)"},{"comment":"The text says 'F2^p/F2^d is taken from Table 2 of Ref. [36]' and then immediately says 'We determine all parameters, including those of the UMF and F2^p/F2^d simultaneously from data.' Please clarify whether Table 2 is used only for priors or initial values, and whether deuteron data themselves enter the simultaneous fit.","section":"Sec. II, after Eq. (4)"},{"comment":"The caption states that predictions for 2F2^{3H}/3F2^d are based on the assumption n^{3H}_SRC = n^{3He}_SRC, but the sensitivity of this assumption is only reported in the supplementary materials. Since this is a model input for the MARATHON prediction, the caption should state the quoted ±20% variation changes the results by less than 5% at moderate and high x.","section":"Sec. III, Fig. 3 caption"},{"comment":"The caption should specify the Q2 evolution applied to each comparison extraction (CT14, CJ15, Arrington et al.) in addition to the overall statement that all extractions were evolved to Q2 = (14 GeV^2) × xB, so that the comparison is reproducible.","section":"Fig. 2 caption"}],"recommendation":"major_revision","confidential_remarks":"The paper addresses an important question and the analysis is carefully executed within its chosen SRC framework, but the central claim depends on a universality assumption that is not tested against conventional non-SRC nuclear models. I would like to see a quantitative closure test before publication; if the test shows only small shifts, the result would be considerably more convincing, while if it shows a large shift the conclusion may need to be revised. I have no concerns about citation practices or scope."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"You should know this paper before the MARATHON data drops. It is the first extraction of the free neutron structure function ratio that uses a global fit to nuclear DIS from deuterium to lead, with a single universal modification function tied to SRC pair abundances. The method is not circular in the cheap way: F2^n/F2^p is algebraically derived from funiv and F2^p/F2^d, so the central number is not put in by hand. The fit reproduces the F2^A/F2^d data, the robustness checks (removing low-W data, Q2 evolution) do not destroy the saturation at xB≈0.6, and the result agrees with the tagged deuterium BONuS points. That is a real piece of work, and the predictions for the MARATHON ratio and the correction function R are the right outputs.\n\nThe soft spot is exactly where the stress test lands. funiv is defined relative to F2^d, and the same funiv is then used to correct deuterium and extract F2^n/F2^p. If deuterium has a high-x tail from Fermi motion, binding, or off-shell effects that does not scale with a2(A/d), that deuteron-specific behavior gets absorbed into funiv, and the extracted ratio will be biased precisely in the xB≥0.6 region where the 0.47±0.04 limit is claimed. The authors state this assumption plainly in Sec. II, but they do not test it against a non-SRC Fermi-motion model. So the quoted 0.04 is the posterior width of their parameterization, not the model uncertainty. I also note that the 3He/3H prediction partially reuses 3He data that constrained funiv, and the tritium n_SRC is assumed equal to 3He; those are minor issues, but the first means the MARATHON comparison is less independent than it looks.\n\nNone of this makes the paper unserious. The central claim is a model-dependent result, not a measurement, and the authors are mostly transparent about the key assumption. But if I were refereeing, I would ask them to quantify the model uncertainty explicitly, ideally with a closure test using a Fermi-motion smearing model, and to share the code and data. The paper deserves a serious referee and will be cited regardless, because it sets the stage for MARATHON and pins down what is at stake in the deuterium correction.","headline":"A new and internally coherent extraction of F2^n/F2^p from A>2 nuclear DIS, with a central claim that holds together but whose 0.47±0.04 is only as robust as the universal SRC modification assumption.","tokens_in":11009,"tokens_out":1846,"would_cite":true,"duration_ms":22091,"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":"Nuclear deep-inelastic data put the neutron-to-proton structure-function ratio at 0.47 ± 0.04 as momentum fraction approaches one.","keywords":["neutron structure function","deep inelastic scattering","EMC effect","short-range correlations","universal modification function","neutron-to-proton ratio","spin-flavor SU(6) breaking","MARATHON experiment"],"falsifier":"The direct test is the published helium-3/tritium $F_2$ ratio from the A=3 mirror-nuclei experiment: inserting that ratio into Eq. 6 with an independently validated $R$ must reproduce $F_2^n/F_2^p = 0.47 \\pm 0.04$ as $x_B$ approaches 1. A result consistent with $1/4$ or with $2/3$ at the same kinematics, or outside roughly 0.43–0.51, would falsify the extraction; a higher-precision tagged-deuterium measurement of the ratio for $x_B > 0.6$ provides an independent check of the same plateau.","tokens_in":9888,"feed_emoji":"⚛️","tokens_out":13264,"duration_ms":122723,"temperature":0.7,"pith_summary":"The paper aims to settle which mechanism breaks spin-flavor SU(6) symmetry, the quark-model symmetry that treats up and down quarks and their spin states as interchangeable, by extracting the free neutron structure function from deep-inelastic scattering data on nuclei from deuterium to lead. Because there is no free neutron target, the extraction requires correcting for the nuclear modification of bound-nucleon quark distributions (the EMC effect); the paper does this with a single universal modification function tied to short-range correlated proton-neutron pairs. The central result is that the neutron-to-proton structure-function ratio $F_2^n/F_2^p$ becomes constant for $x_B \\ge 0.6$ and equals $0.47 \\pm 0.04$ in the $x_B \\to 1$ limit. This matches perturbative QCD ($3/7$) and Dyson-Schwinger predictions ($0.41$–$0.49$) and contradicts the scalar diquark prediction ($1/4$). If correct, the result also sharpens the interpretation of the recently measured helium-3 / tritium ratio and quantifies the nuclear correction needed to turn it into a neutron measurement.","feed_headline":"Neutron structure ratio saturates at 0.47","feed_subtitle":"The extracted limit backs perturbative QCD and Dyson-Schwinger over the scalar diquark model.","key_machinery":"The load-bearing object is the universal modification function $f_{univ}(x_B)$, the average change in the structure function of a nucleon inside a short-range correlated pair normalized to the deuteron (Eq. 3). It enters Eq. 1, which writes any nuclear structure function as $F_2^A = Z F_2^p + N F_2^n + n_{SRC}^A(\\Delta F_2^p + \\Delta F_2^n)$: unmodified mean-field nucleons plus modified correlated nucleons. The assumption that the EMC effect for $0.3 \\lesssim x_B \\lesssim 0.7$ and nucleon-motion effects for $x_B > 0.7$ are both proportional to the number of nucleons in proton-neutron SRC pairs makes $f_{univ}$ independent of the nucleus. The analysis extracts it from $F_2^A/F_2^d$ ratios for eight nuclei using Bayesian Hamiltonian Markov Chain Monte Carlo, then converts the deuterium ratio into the neutron-to-proton structure-function ratio via Eq. 5, $F_2^n/F_2^p = (1 - f_{univ})/(F_2^p/F_2^d) - 1$, and generates the A=3 predictions through the correction factor $R$ of Eq. 7.","core_discovery":"The paper's central discovery is that $F_2^n/F_2^p$ saturates for $x_B \\ge 0.6$ at $0.47 \\pm 0.04$ as $x_B$ approaches 1. The authors reach this by writing every nuclear structure function as unmodified mean-field nucleons plus modified nucleons inside short-range correlated proton-neutron pairs, with one nucleus-independent universal modification function; fitting that function to $F_2^A/F_2^d$ data from helium-3 to lead then corrects the deuterium data from which the neutron ratio is read off. On the paper's own terms this is decisive: the extracted limit agrees with perturbative QCD ($3/7$) and the Dyson-Schwinger range ($0.41$–$0.49$), and disagrees with the scalar-diquark prediction ($1/4$) and the SU(6) value ($2/3$). Previous deuterium-only extractions either could not distinguish these predictions or favored the scalar diquark, so the wider nuclear lever arm is what changes the answer. The same framework predicts the helium-3/tritium ratio from the A=3 mirror-nuclei experiment and shows that different models of the nuclear correction factor $R$ shift the extracted neutron ratio by up to about 25% at high $x_B$.","pith_inferences":["If the plateau survives the A=3 and tagged-deuterium checks, large-$x$ parton distributions will need re-tuning: the implied $d/u$ ratio at $x$ near 1 enters $W^\\pm$ and $Z$ production rates at colliders, so the change propagates beyond nuclear physics.","The universality of $f_{univ}$ is itself a testable nuclear-physics statement: tagged-spectator measurements at high $x_B$ can check that the per-pair modification inferred from heavy nuclei equals that seen in the deuteron, exposing where short-range-correlation dominance of the EMC effect breaks down.","With $F_2^n/F_2^p$ fixed, inclusive DIS ratios of less-studied nuclei become constraints on their short-range-correlation content, turning the neutron into a calibrated probe of nuclear structure.","The central value sits above the perturbative-QCD value 3/7 and within the upper part of the Dyson-Schwinger range, so improved A=3 precision could separate those two surviving predictions."],"forward_implications":["The high-$x_B$ neutron-to-proton structure-function ratio is fixed at a plateau of $0.47 \\pm 0.04$ for $x_B \\ge 0.6$, giving a target that future valence-quark extractions must reproduce.","The scalar diquark dominance picture, with limit $1/4$, is excluded by the combined nuclear data set; perturbative QCD ($3/7$) and Dyson-Schwinger ($0.41$–$0.49$) predictions survive.","The universal modification function from the global fit predicts the helium-3/tritium cross-section ratio and the correction function $R$ that converts that ratio into $F_2^n/F_2^p$; different models of $R$ change the extracted neutron ratio by up to about 25% at high $x_B$.","Including nuclei heavier than deuterium is what separates this result from deuterium-only global fits, which either could not discriminate among the models or favored the scalar diquark.","Removing low-energy resonance-region data or evolving the proton-to-deuteron ratio between $Q^2$ values does not change the saturation for $x_B$ up to about 0.8."],"supporting_citations":[{"why":"Supplies the $F_2^A/F_2^d$ data and the per-nucleus universal-modification extractions that the global fit reproduces and extends.","marker":"[14]"},{"why":"Provides the $a_2(A/d)$ short-range-correlation abundance ratios that set the $n_{SRC}^A$ weights in Eq. (1).","marker":"[12]"},{"why":"States the scale-separation argument that EMC and high-$x$ Fermi-motion effects are both dominated by SRC pairs.","marker":"[15]"},{"why":"Provides the $F_2^p/F_2^d$ parameterization that the fit determines simultaneously with the UMF.","marker":"[36]"},{"why":"Tagged-deuterium DIS data point used as the experimental cross-check of the extracted neutron-to-proton ratio.","marker":"[37]"},{"why":"The A=3 mirror-nuclei experiment whose $F_2^{^3He}/F_2^{^3H}$ ratio and correction function $R$ the paper predicts.","marker":"[9]"},{"why":"Dyson-Schwinger prediction ($0.41$–$0.49$) that the extracted $x_B \\to 1$ limit is compared with.","marker":"[2]"},{"why":"Perturbative-QCD prediction ($3/7$) that the extracted limit agrees with.","marker":"[3]"},{"why":"Scalar-diquark prediction ($1/4$) that the extracted limit disagrees with.","marker":"[7]"}],"fun_headline_variants":["Neutron ratio saturates at 0.47, backs pQCD","High-x neutron ratio locks at 0.47","Neutron structure limit settles QCD debate","Nuclear data pin neutron ratio to 0.47"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"Everything rests on assuming that a single universal modification function, proportional to the fraction of nucleons in proton-neutron short-range correlated pairs, describes the nuclear modification of structure functions, including Fermi-motion effects above $x_B = 0.7$, for every nucleus from deuterium to lead; if that universality fails, the deuterium correction is biased and the extracted limit is not the free-neutron value.","fun_headline_variants_meta":{"raw":{"variants":["Neutron ratio saturates at 0.47, backs pQCD","High-x neutron ratio locks at 0.47","Neutron structure limit settles QCD debate","Nuclear data pin neutron ratio to 0.47"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000446,"raw_usage":{"total_tokens":2326,"prompt_tokens":1089,"completion_tokens":1237,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":705,"completion_tokens_details":{"reasoning_tokens":1168}},"tokens_in":705,"tokens_out":1237,"duration_ms":11471,"temperature":1.0,"reasoning_tokens":1168,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-14T14:51:03.745177+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"The direct test is the published helium-3/tritium $F_2$ ratio from the A=3 mirror-nuclei experiment: inserting that ratio into Eq. 6 with an independently validated $R$ must reproduce $F_2^n/F_2^p = 0.47 \\pm 0.04$ as $x_B$ approaches 1. A result consistent with $1/4$ or with $2/3$ at the same kinematics, or outside roughly 0.43–0.51, would falsify the extraction; a higher-precision tagged-deuterium measurement of the ratio for $x_B > 0.6$ provides an independent check of the same plateau.","supporting_citations":[{"cited_title":"Schmookler et al","cited_arxiv_id":null,"evidence_quote":"Supplies the $F_2^A/F_2^d$ data and the per-nucleus universal-modification extractions that the global fit reproduces and extends."},{"cited_title":"Frankfurt and M","cited_arxiv_id":null,"evidence_quote":"States the scale-separation argument that EMC and high-$x$ Fermi-motion effects are both dominated by SRC pairs."},{"cited_title":"Neutron Structure Functions","cited_arxiv_id":"0805.3116","evidence_quote":"Provides the $F_2^p/F_2^d$ parameterization that the fit determines simultaneously with the UMF."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"The A=3 mirror-nuclei experiment whose $F_2^{^3He}/F_2^{^3H}$ ratio and correction function $R$ the paper predicts."},{"cited_title":"Nucleon spin structure at very high-x","cited_arxiv_id":"1308.1236","evidence_quote":"Dyson-Schwinger prediction ($0.41$–$0.49$) that the extracted $x_B \\to 1$ limit is compared with."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Perturbative-QCD prediction ($3/7$) that the extracted limit agrees with."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Scalar-diquark prediction ($1/4$) that the extracted limit disagrees with."}],"review_version":1}