{"id":"5dcb9cfa-c101-4200-aad0-e59896b70432","arxiv_id":"2509.17123","paper_version":2,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":3,"one_line_summary":"A simulation study shows that combining a tagged neutron's direction with missing transverse momentum improves neutrino energy reconstruction in several channels, though the gains are channel-dependent.","lead":"This paper tests a way to estimate a neutron's energy in neutrino collisions by combining the neutron's measured direction with the missing sideways momentum of the other particles. If the idea survives real detector tests, it could improve neutrino energy measurements at DUNE, T2K, and SBN.","discovery_kind":"new_method","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The method's central gain depends on the unsmeared assumption that the tagged neutron direction equals the vertex-to-deposit vector (Eq. 1, Sec. III A); no neutron transport or scattering is modeled, so the reported Tn and Eν improvements may be optimistic.","rationale":"I read the paper as a generator-level methods study whose central claim is that the direction of a tagged neutron, combined with missing transverse momentum, yields a better-centered neutron kinetic energy estimate and improves neutrino energy reconstruction. The reader's weakest assumption identifies the direction assumption, and I agree that this is the most load-bearing concern. Equation (1) converts a transverse momentum component into a full neutron momentum using the geometric factor |R_n|/|R_T,n| = 1/sin(theta); if the tag position does not reflect the initial neutron direction, this factor is wrong event by event. The paper explicitly does not smear or transport the neutron direction, so the simulation is run in the most favorable regime for the method. The fact that the new Tn sigma' is worse than the pT-balance method in all four Table II rows makes the E_nu improvement subtle and potentially fragile. A realistic transport or angular-smearing test would settle whether the improvement survives. Because the paper is clearly framed as an initial methods study, states its assumptions, and points to a reproducible container, I would not reject it, but it should not be accepted as a final demonstration without this test. The reader's CONDITIONAL verdict is therefore appropriate and unchanged.","tokens_in":7146,"tokens_out":11639,"duration_ms":111498,"concrete_test":"Run the same GENIE events through a Geant4 neutron-transport step in LAr and CH (or, minimally, apply Gaussian smearing to the neutron direction with sigma_theta = 5, 15, and 30 degrees), take the first inelastic interaction as the tag, and recompute Table II with the same Delta_theta_T < 30 deg cut and mean-bias correction. Also recompute for an exclusive subsample with exactly one neutron above 20 MeV. If the E_nu sigma' advantage over the pT-balance method shrinks by more than about 20% or reverses, the central claim is not robust to realistic neutron direction and missing-momentum assumptions.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central estimator (Eq. 1) requires that the initial neutron momentum points along R_n (vertex to tagged deposition) and that p_T,miss,reco is essentially the tagged neutron's transverse momentum. Section III A states that the spatial vector from the neutrino interaction point to the tagged energy deposition was assumed to give a good measure of the initial neutron direction and that 'this quantity was not smeared.' Real neutrons scatter before depositing energy, especially in argon, so the tag position need not point along the initial neutron direction. The paper also ignores all other produced neutrons when forming p_T,miss,reco, so the second premise fails in multi-neutron events. Table II shows that the new Tn sigma' is larger (worse) than the pT-balance method in every channel, so the reported neutrino-energy gain is not a tighter Tn estimate but a better-centered median; direction errors from scattering would directly attack that centeredness, and multi-neutron contamination changes the interpretation of p_T,miss,reco. These are self-flagged limitations (Sec. III A: 'This quantity was not smeared'; Sec. IV: 'would benefit from a more sophisticated simulation'), but they are not tested.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper proposes a method to estimate the kinetic energy of a tagged neutron in charged-current neutrino interactions by combining the direction from the interaction vertex to the tagged energy deposition with the reconstructed missing transverse momentum. Equation (1) sets the neutron momentum magnitude using the component of the missing transverse momentum parallel to the projected neutron direction. The method is evaluated with GENIE simulations for carbon and argon targets under T2K-like and DUNE-like fluxes, with nominal detection thresholds and momentum/angular smearing; the resulting neutron kinetic energy is added to a calorimetric sum to reconstruct the neutrino energy. The authors report improved centeredness and, for most channels, improved width (sigma') of the reconstructed neutrino energy, and they suggest that a cut on the opening angle Delta_theta_T can enrich a sample of interactions on hydrogen. The paper is a generator-level methods study with explicit acknowledgement that a more sophisticated simulation, including backgrounds and detector effects, is needed.","tokens_in":7376,"tokens_out":4724,"duration_ms":44511,"significance":"If the technique survives a more realistic simulation, it would be a useful and inexpensive addition to neutron-inclusive reconstruction in liquid argon and scintillator detectors, complementing time-of-flight methods and providing a way to include a fraction of produced neutrons in event-by-event neutrino energy reconstruction. The paper is commendable for defining a robust non-Gaussian width metric, making generator and flux choices explicit, and comparing against two baseline algorithms. However, the quantitative claims are conditionally supported by an idealized simulation: the neutron direction is taken as the unsmeared vertex-to-deposit vector, non-tagged neutrons are ignored in the missing transverse momentum, and the mean kinetic energy bias is corrected using the same simulated sample that is used to evaluate the resolution. These limitations make the reported resolutions optimistic and model-dependent, so the significance of the result is not yet established at the level claimed in the abstract.","major_comments":[{"comment":"The central estimator assumes that the initial neutron direction is equal to the spatial vector from the neutrino interaction point to the tagged energy deposition, and the text states 'This quantity was not smeared.' Real neutrons can scatter before depositing energy, especially in argon and at higher kinetic energies, so the vertex-to-deposit vector need not point along the initial neutron direction. Because this assumption directly feeds Eq. (1), the reported Tn and neutrino-energy resolutions are likely optimistic. I request a sensitivity study that smears or perturbs the neutron direction according to a scattering model, or a generator-level study including final-state interactions and neutron transport, with the effect on the quoted sigma' values quantified.","section":"Sec. III A, Eq. (1)"},{"comment":"The study selects only the highest-kinetic-energy neutron as the tagged candidate and states that 'Produced neutrons, other than the one considered to be tagged, were ignored as undetected particles.' In events with more than one produced neutron, the reconstructed missing transverse momentum p_T,miss,reco contains contributions from all unreconstructed particles, not just the tagged neutron, so the identification of p_T,miss,reco with the tagged neutron's transverse momentum fails. The paper should report the fraction of selected events with additional neutrons above the detection threshold and show how the Tn and neutrino-energy resolutions change when such events are excluded or treated differently.","section":"Sec. III A"},{"comment":"The claim that including the neutron improves the neutrino energy resolution is not uniform across channels. For nu_mu on carbon, the full correction yields sigma' = 0.151 for E_nu, slightly worse than the no-correction value of 0.149; for the Tn reconstruction, the full-direction method has a larger sigma' than the transverse-balance method in every channel (for example, 1.340 versus 0.781 for nu_mu on carbon). The abstract and conclusions emphasize improved resolution and symmetry, but the actual improvement for Tn is in centeredness rather than width. This distinction should be made explicitly, and the channel dependence should be discussed rather than glossed over.","section":"Table II"},{"comment":"The 'correction for the mean kinetic energy bias using the simulation' is an in-sample calibration: a per-channel offset is derived from the same simulated sample that is then used to compute the reported sigma' values. This procedure can artificially center the reconstructed distributions and makes the quoted resolutions dependent on the generator tune and flux model. The authors should specify how many parameters are fitted, how large the offsets are relative to the signal, and ideally validate the correction on an independent simulated sample or with a different generator configuration.","section":"Sec. III B"}],"minor_comments":[{"comment":"The sentence beginning 'Under the assumption that p_T,miss,reco cos Delta_theta_T ...' is grammatically incomplete and should be rewritten to state that the magnitude |p_T,miss,reco| cos Delta_theta_T is taken to equal the transverse momentum of the tagged neutron.","section":"Sec. II"},{"comment":"The notation p_T,n is used both as a vector and as a magnitude; please define it explicitly as a magnitude to avoid confusion.","section":"Eq. (1)"},{"comment":"The column labels 'No corr.', 'Trans. corr.', and 'Full corr.' are not defined in the caption or in the text; please define them clearly and relate them to the three algorithms shown in Fig. 5.","section":"Table II"},{"comment":"The figures would benefit from self-contained legends and captions that identify the solid, hatched, and dotted histograms; currently the reader must infer the correspondence from the text.","section":"Figs. 3-6"},{"comment":"The definition of sigma' as the 'half-width of the length that would encompass the central 68% of the events' is ambiguous for asymmetric distributions; please specify whether the quoted values are the average of the two half-widths or the full width divided by two.","section":"Sec. III B"},{"comment":"No statistical uncertainties are quoted for the sigma' values; given the finite simulated sample sizes, a statement about statistical precision would be useful.","section":"Tables and figures"}],"recommendation":"major_revision","confidential_remarks":"The paper is an exploratory methods study with a promising core idea, but the simulation is too idealized to support the quantitative claims as currently presented. The authors should be encouraged to add a neutron-scattering sensitivity study or a more realistic detector-level simulation, and to address the in-sample calibration issue. With those additions, the paper could become suitable for publication."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"What you should know: this paper proposes a new kinematic estimator—Eq. (1)—that combines the 3D direction of a tagged neutron with the missing transverse momentum to infer the neutron's kinetic energy and fold it into the reconstructed neutrino energy. The combination is genuinely new, building on Myatt's old transverse-balance idea but using the tagged neutron's direction instead of the total hadronic momentum. The paper is clearly written, the estimator is sensible, and the authors are honest about several limitations. It is a methods study, not a final demonstration.\n\nWhat it does well: the GENIE-based setup is straightforward, the detection thresholds and smearing are stated, and they define a robust width metric σ′ for non-Gaussian distributions. They also report channel-dependent results rather than cherry-picking. The idea that a small ΔθT cut could enrich hydrogen interactions in CH is interesting and plausible. The paper explicitly flags that a more sophisticated simulation with backgrounds and better detector effects is needed, which is credit where it is due.\n\nThe soft spots are real but proportionate. The load-bearing assumption, stated in Sec. III A, is that the vector from the neutrino vertex to the tagged energy deposition equals the neutron's initial direction, and that this quantity was not smeared. Real neutrons scatter before depositing energy, especially in argon. The stress-test note is right: this is the main reason the reported gains may be optimistic. The paper does not model neutron transport or multi-neutron events, where the other neutrons corrupt pT,miss,reco.\n\nMore telling: Table II shows that the new method's Tn resolution σ′ is worse than the pT-balance method in every channel (e.g., 1.340 vs 0.781 for νμ on C). The neutrino energy resolution improves not because the Tn estimate is tighter, but because it is better centered after a per-channel mean bias correction fitted to the same simulation used for scoring. That circularity is not fatal for a methods study, but it means the reported Eν gains are not yet evidence of a robust improvement. No error bars, background model, or code are provided.\n\nBottom line: the core idea is worth taking seriously, but the central claims as stated overreach the simulation. A serious referee should push for a validation on a separate sample, a realistic neutron-scattering model, and a clearer statement of which channels actually benefit. For someone working on neutron reconstruction in LArTPCs or SuperFGD, this is a useful starting point.\n\nRecommendation: send it to peer review. It deserves referee time, even though heavy revision is likely.","headline":"A plausible new estimator for neutron kinetic energy that improves reconstructed Eν in simulation, but the gains rest on an untested no-scattering direction assumption and a per-channel bias correction fitted on the same sample.","tokens_in":7965,"tokens_out":1569,"would_cite":true,"duration_ms":15937,"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 paper claims that combining a tagged neutron's direction with missing transverse momentum recovers the neutron's kinetic energy and noticeably improves reconstructed neutrino energy resolution.","keywords":["neutrino energy reconstruction","neutron tagging","neutron kinetic energy","missing transverse momentum","liquid argon detectors","scintillator detectors","charged-current neutrino interactions","Monte Carlo simulation"],"falsifier":"In a Monte Carlo that records both the true initial neutron direction and the direction at the energy deposit, compare the Eq. (1) $T_n$ with the true $T_n$ for events where the neutron scatters before depositing; if the reconstructed $T_n$ is biased exactly in those events, the no-scatter assumption is the cause. Equivalently, in a detector with independent time-of-flight neutron energy measurements, check whether events with large $\\Delta\\theta_T$ show the predicted loss of accuracy.","tokens_in":6890,"feed_emoji":"⚛️","tokens_out":11087,"duration_ms":85825,"temperature":0.7,"pith_summary":"This paper proposes a way to include neutrons in event-by-event neutrino energy reconstruction, something accelerator neutrino experiments have struggled to do. The idea is to combine the direction of a detected neutron energy deposit, pointing back to the neutrino interaction vertex, with the missing transverse momentum of the reconstructed charged particles. A simple ratio identity then gives the neutron's momentum magnitude and kinetic energy, and adding that estimate to the calorimetric sum improves the reconstructed neutrino energy resolution and makes its distribution more symmetric about the true value. In the paper's simulation, the neutrino energy width drops from about 16% to 10% for muon neutrinos on argon, and from about 11% to 6.5% for antineutrinos on argon.","feed_headline":"Using tagged neutron directions sharpens neutrino energy resolution","feed_subtitle":"A vertex-to-tag direction plus missing momentum recovers neutron energy and centers the neutrino-energy peak","key_machinery":"The load-bearing object is the lever-arm ratio identity of Eq. (1), $|\\vec{p}_n| = |\\vec{p}_{T,n}|\\,|\\vec{R}_n|/|\\vec{R}_{T,n}|$, which converts a transverse-plane momentum balance into a full three-dimensional neutron momentum. It is used in two steps: first project the missing transverse momentum onto the transverse direction of the tagged neutron to get $|\\vec{p}_{T,n}|$; then scale by the ratio of the full vertex-to-tag distance to its transverse projection, which accounts for motion out of the transverse plane. The identity's validity rests on the tagged neutron being the dominant missing transverse momentum and on the vertex-to-tag direction approximating the neutron's initial direction. The angle $\\Delta\\theta_T$ between $\\vec{p}_{T,\\mathrm{miss}}$ and $\\vec{R}_{T,n}$ functions as a diagnostic of when those assumptions break down.","core_discovery":"The paper's central claim is that when a neutron produced in a charged-current neutrino interaction leaves a detectable energy deposit, the three-dimensional vector from the interaction vertex to that deposit is a serviceable stand-in for the neutron's initial direction, and that direction is enough to turn the usual missing-transverse-momentum imbalance into an event-by-event estimate of the neutron's kinetic energy. Concretely, with $\\vec{R}_n$ the vertex-to-tag vector and $\\vec{R}_{T,n}$ its component transverse to the neutrino beam, the component of the missing transverse momentum $\\vec{p}_{T,\\mathrm{miss}}$ parallel to $\\vec{R}_{T,n}$ is taken to be the neutron's transverse momentum, and the full momentum magnitude follows from the lever-arm identity $|\\vec{p}_n| = |\\vec{p}_{T,n}|\\,|\\vec{R}_n|/|\\vec{R}_{T,n}|$. From $|\\vec{p}_n|$ the neutron kinetic energy $T_n$ follows using the known neutron mass. The paper argues, using generator-level simulations with detector-like thresholds and smearing, that including this $T_n$ in the calorimetric neutrino energy sum yields a reconstructed neutrino energy that is more centered on the true value and has a narrower width: the width measure $\\sigma'$ drops from 15.9% to 9.8% for $\\nu_\\mu$ on argon and from 10.9% to 6.5% for $\\bar{\\nu}_\\mu$ on argon. The same idea is shown to improve the neutron kinetic energy estimate relative to simply identifying $|\\vec{p}_n|$ with $|\\vec{p}_{T,\\mathrm{miss}}|$, and a cut on the angle $\\Delta\\theta_T$ between the two vectors can enrich hydrogen interactions and suppress flat backgrounds.","pith_inferences":["Because the method is most reliable at small $\\Delta\\theta_T$, that same angle could serve as a per-event quality flag or event weight, providing a simple way to suppress biased events without a full unfolding.","The underlying lever-arm identity is not specific to neutrons; a similar vertex-to-tag direction plus transverse-momentum projection could constrain the energy of other neutral final-state particles that leave a localized energy deposit.","The paper itself notes that a more sophisticated simulation including backgrounds, detection inefficiencies, and better detector effects is needed, especially for hydrogen interactions; the size of the resolution gain in a realistic analysis remains an open question.","Where independent time-of-flight neutron energy is available, combining it with the direction-based estimate would give a cross-check of the no-scatter assumption and potentially a joint estimator with better resolution than either method alone."],"forward_implications":["Detectors that can tag a neutron's three-dimensional energy deposition and point it back to the interaction vertex can reconstruct that neutron's kinetic energy event by event, without needing time-of-flight.","In the paper's simulation, including the reconstructed neutron narrows the argon neutrino energy width from $\\sigma'=15.9\\%$ to $9.8\\%$ for $\\nu_\\mu$ and from $10.9\\%$ to $6.5\\%$ for $\\bar{\\nu}_\\mu$.","A cut on the angle $\\Delta\\theta_T$ between the missing transverse momentum and the neutron's transverse direction can enrich samples of neutrino interactions on hydrogen and suppress flat backgrounds.","The approach is compatible with liquid argon detectors and with the three-dimensional scintillator detector in the T2K near detector, and it can complement time-of-flight neutron energy measurements.","The largest relative improvement in reconstructed neutrino energy width appears for antineutrino interactions, where neutron production is more important."],"supporting_citations":[{"why":"Demonstrates the neutron-tagging capability in a liquid argon TPC that supplies the direction measurement this method builds on.","marker":"[12]"},{"why":"Provides the time-of-flight neutron energy reconstruction that this direction-based method is compared with as an alternative or complement.","marker":"[10]"},{"why":"The earlier transverse-momentum-balance method for estimating the energy of escaping neutral particles, which this paper extends by using the tagged neutron direction.","marker":"[13]"},{"why":"Supplies the simulated neutrino events used to assess the technique at generator level.","marker":"[14]"},{"why":"Provides the software framework used to convert the generated event output into analysis trees.","marker":"[15]"},{"why":"Supplies the DUNE-like beam flux spectra used for the argon simulations.","marker":"[17]"},{"why":"Supplies the T2K-like beam flux spectra used for the carbon and scintillator simulations.","marker":"[18]"},{"why":"Describes the three-dimensional scintillator detector whose neutron-tagging and timing capabilities motivate the technique.","marker":"[9]"}],"fun_headline_variants":["Neutron direction boosts neutrino energy reconstruction","Tagged neutron vectors improve neutrino energy resolution","Neutron direction centers neutrino energy peak","Direction of tagged neutrons sharpens neutrino energy","Neutron track direction enhances neutrino energy estimate"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The method assumes the straight line from the neutrino vertex to the tagged neutron energy deposit is the neutron's initial direction, with no significant scattering before the deposit and no smearing of that direction.","fun_headline_variants_meta":{"raw":{"variants":["Neutron direction boosts neutrino energy reconstruction","Tagged neutron vectors improve neutrino energy resolution","Neutron direction centers neutrino energy peak","Direction of tagged neutrons sharpens neutrino energy","Neutron track direction enhances neutrino energy estimate"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000457,"raw_usage":{"total_tokens":2363,"prompt_tokens":1083,"completion_tokens":1280,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":699,"completion_tokens_details":{"reasoning_tokens":1227}},"tokens_in":699,"tokens_out":1280,"duration_ms":8640,"temperature":1.0,"reasoning_tokens":1227,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-15T15:47:52.841306+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"In a Monte Carlo that records both the true initial neutron direction and the direction at the energy deposit, compare the Eq. (1) $T_n$ with the true $T_n$ for events where the neutron scatters before depositing; if the reconstructed $T_n$ is biased exactly in those events, the no-scatter assumption is the cause. Equivalently, in a detector with independent time-of-flight neutron energy measurements, check whether events with large $\\Delta\\theta_T$ show the predicted loss of accuracy.","supporting_citations":[{"cited_title":"Abratenkoet al.(MicroBooNE Collaboration), Demonstration of neutron identification in neutrino interactions in the MicroBooNE liquid argon time projection chamber, Eur","cited_arxiv_id":null,"evidence_quote":"Demonstrates the neutron-tagging capability in a liquid argon TPC that supplies the direction measurement this method builds on."},{"cited_title":"Munteanuet al., New method for an improved antineutrino energy reconstruction with charged-current interactions in next-generation detectors, Phys","cited_arxiv_id":null,"evidence_quote":"Provides the time-of-flight neutron energy reconstruction that this direction-based method is compared with as an alternative or complement."},{"cited_title":"Myatt, The Use of Transverse Momentum Balance as a Means of Estimating the Energy of Neutrino Interactions, CERN/EFCA/72-4 (1972)","cited_arxiv_id":null,"evidence_quote":"The earlier transverse-momentum-balance method for estimating the energy of escaping neutral particles, which this paper extends by using the tagged neutron direction."},{"cited_title":"Abeet al.(T2K Collaboration), T2K neutrino flux prediction, Phys","cited_arxiv_id":null,"evidence_quote":"Supplies the T2K-like beam flux spectra used for the carbon and scintillator simulations."}],"review_version":2}