{"id":"83152b2b-336e-4f28-9e0e-2b2e2c140072","arxiv_id":"2501.12871","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":5.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":7,"one_line_summary":"A model-independent analysis of Lambda_c to (Lambda, neutron) l nu decays finds SM rates about 10 percent above experiment and proposes the muon-to-electron forward-backward asymmetry ratio as a robust new physics probe.","lead":"This paper calculates the Standard Model and new physics predictions for the charm-baryon decays Lambda_c to Lambda l nu and Lambda_c to neutron l nu, using lattice QCD form factors and a full set of four-fermion operators. It finds the SM branching fractions run about 10 percent above current BESIII central values, and it proposes a ratio of forward-backward asymmetries as a cleaner new-physics test.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The 'up to 10%' NP suppression rests on an ad hoc benchmark CSR=0.3, not on the fitted D/Ds coefficients the abstract invokes; a realistic fitted value would shrink the effect to a few percent.","rationale":"The reader correctly identifies the lattice form factors as an external input on which all numerical results depend, and that concern is legitimate. However, the form factors are independently published lattice results whose central values and uncertainties can be checked directly against Refs. [21] and [45]; the same is true of the SM branching-fraction comparison. The more load-bearing weakness is internal to the paper's NP claim: the manuscript itself discloses that the Wilson coefficients in Eq. (102) were chosen to maximize NP effects rather than taken from a fit, while the abstract describes them as fitted. Since the scalar-operator suppression is approximately linear in CSR, using 0.3 instead of the O(10^-2)-O(10^-1) values quoted from the fit inflates the central NP statement by a large factor. The added internal sign inconsistency in the summary makes the NP direction ambiguous rather than a clean prediction. None of this invalidates the general framework, the SM results, or the ratio A_mu/e_FB as a future observable, but it does mean the paper's most falsifiable NP claim should be presented as an extreme benchmark, conditional on the fitted bound on CSR. The reader's CONDITIONAL verdict remains appropriate, so no change to the verdict is required.","tokens_in":30048,"tokens_out":10300,"duration_ms":115430,"concrete_test":"Re-run the global chi-squared fit of Ref. [18] (or Ref. [22]) with current BESIII and PDG data for D_s -> mu nu, D -> K(mu nu), D_s -> phi mu nu, treating CSR as a free real coefficient, and extract its 95% CL upper bound. Then recompute the Lambda_c -> Lambda mu nu branching fraction relative to the SM using Eqs. (66)-(84) with that bound and the same lattice form factors. If the bound is below about 0.1, the predicted suppression drops below roughly 3%, i.e. inside the SM uncertainty, and the 'up to 10%' NP claim is not supported; if the fit allows CSR=0.3, the claim survives. A quick analytic cross-check is to scale the reported 10% suppression linearly by the ratio of the fitted bound to 0.3.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central NP claim is not actually tied to the fitted Wilson coefficients. In Sec. 3.3 the authors report that the D/Ds fits of Ref. [18] give CVL,R of order 10^-3 and CSL,SR of order 10^-2, and that Ref. [56] allows scalar coefficients at most of order 10^-1; yet Eq. (102) adopts CVL=0.03, CVR=-0.01, CSL=CSR=0.3, CT=0.15 'in order to maximize manifestation of NP effects.' The abstract nonetheless states that the coefficients were 'fitted from D and D_s meson decays.' Because the scalar contribution enters through a VL-SR interference that is linear in CSR at leading order, the scale of the suppression is set directly by the chosen coefficient: 0.3 is 3-30 times larger than the quoted fitted/allowable values. The headline 'up to 10%' suppression is therefore an upper-bound illustration, not a prediction, and a realistic CSR would move the NP effect inside the form-factor uncertainties quoted in Eqs. (92)-(99). A separate internal inconsistency strengthens this concern: the summary states that the SM overestimate favors a negative CSR, while Eq. (102) and the figures use positive CSR=0.3 to produce the suppression.","agreement_with_reader":"partial"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper studies the semileptonic charm-baryon decays Λ_c → (Λ, n)ℓ^+ν_ℓ (ℓ = μ, e) in the Standard Model and in a model-independent effective Hamiltonian with all four-fermion operators. The hadronic helicity amplitudes are derived including interference terms between different NP operators, and the numerical input is the lattice QCD z-expansion form factors of Refs. [21,45]. The SM branching fractions, Eqs. (92)-(93) and (98)-(99), are about 10% above the central experimental values for the Λ modes, although consistent within uncertainties. In the NP section, the paper adopts the Wilson coefficients of Eq. (102), claims that the right-handed scalar operator can suppress the muonic modes by up to 10%, and proposes the ratio of forward-backward asymmetries A_B^{μ/e} as a novel probe that is robust against hadronic uncertainties and largely unaffected by the considered NP operators.","tokens_in":30328,"tokens_out":5837,"duration_ms":61532,"significance":"If the SM predictions are taken at face value, the paper provides a useful, technically complete cross-check of lattice QCD form factors and a possible route to improved |V_cs| and |V_cd| determinations. The helicity-amplitude formalism, including all operator interferences in Appendix A, is a valuable reference for future experimental analyses at BESIII, Belle-II, and LHCb. The SM part of the paper is sound and properly uses published lattice input. However, the advertised NP conclusions are currently overstated: the 'up to 10%' suppression is computed at benchmark coefficients that are not the fitted values invoked in the abstract, and the sign preference statement is internally inconsistent. The proposed forward-backward asymmetry ratio needs a clearer logic if it is to be called a probe of NP.","major_comments":[{"comment":"The abstract and summary state that the NP predictions use Wilson coefficients 'fitted from D and D_s meson decays,' but the coefficients adopted in Eq. (102) are not the fit results quoted in Sec. 3.3. The text reports fitted values of order 10^{-3} for C_VL and C_VR and 10^{-2} for C_SL and C_SR, with Ref. [56] allowing at most 10^{-1}, and then sets C_SL = C_SR = 0.3 and C_T = 0.15 'in order to maximize the manifestation of NP effects.' Because the dominant NP contribution enters through the VL-SR interference linearly in C_SR, the quoted 'up to 10%' branching-fraction suppression is an upper-bound benchmark, not a prediction. A coefficient within the fitted or allowable ranges would reduce the effect to a few percent or less, placing it inside the form-factor uncertainties of Eqs. (92)-(99). The NP claims should be reframed as sensitivity illustrations, and the abstract should not describe Eq. (102) as arising from fits.","section":"Sec. 3.3, Eq. (102), abstract"},{"comment":"There is an internal inconsistency in the sign of the right-handed scalar coefficient. The text says that Figure 5 shows the branching fraction 'suppressed by the right-handed scalar operator O_SR,' with Eq. (102) using positive C_SR = 0.3, but later states that the SM overestimate 'implies that the right-handed scalar operator O_SR with a negative Wilson coefficient is much favored.' If a positive C_SR suppresses the rate, a negative coefficient would enhance it, worsening the overestimate; if a negative coefficient suppresses it, the sign used in Eq. (102) and in the figures cannot produce the advertised suppression. The authors must state explicitly the sign of the VL-SR interference term in Eq. (67) and reconcile the benchmark sign with the preferred-sign conclusion.","section":"Sec. 3.3, paragraphs after Fig. 5"},{"comment":"The ratio A_B^{μ/e} of forward-backward asymmetries is presented as a 'novel probe for NP,' yet the paper reports that it is 'largely unaffected by current NP operators.' An observable insensitive to all of the NP operators considered cannot probe those operators; at best it can serve as a cross-check of the SM or as a constraint on operators outside the considered set. The authors should clarify what this observable is intended to probe (for example, lepton-flavor universality in the chiral structure of the current, or NP models with different operator combinations) and specify what size of deviation from the SM would constitute evidence.","section":"Abstract, Sec. 3.3, Sec. 4"}],"minor_comments":[{"comment":"The sentence 'the left-handed scalar operator O_SR introduces slight shape modifications' contains a wrong chirality label; O_SR is the right-handed scalar operator. This appears to be a typo but should be corrected.","section":"Sec. 3.3"},{"comment":"The phrase 'the rest frame of the off-shell Woff − shell' appears garbled; it should read 'the rest frame of the off-shell W' or similar.","section":"Sec. 2.3, around Eq. (65)"},{"comment":"The text says 'It can seen that there is little deviation'; 'can seen' should be 'can be seen.'","section":"Sec. 3.2, after Eq. (97)"},{"comment":"The tables compare many models, but the caption and text do not state which uncertainties are included in each model's errors; for the models without uncertainties (e.g., CQM, RQM, CCQM, NRQM in Table 5) the absence of errors should be noted explicitly.","section":"Table 4 and Table 5"},{"comment":"The caption says 'red line and blue one are for Λ_c^+ → Λ ℓ^+ν_ℓ and Λ_c^+ → nℓ^+ν_ℓ, respectively,' but the color assignment appears reversed relative to the text in Sec. 3.2 ('the blue lines correspond to the decay Λ_c^+ → μℓ^+ν_ℓ, while the red lines represent Λ_c^+ → nℓ^+ν_ℓ'). Please check and harmonize the figure and text.","section":"Figure 4 caption"}],"recommendation":"major_revision","confidential_remarks":"The SM part of the paper is solid and the helicity-amplitude formalism is a useful reference. The main concern is that the headline NP claims, including the abstract's 'fitted coefficients' wording and the 'up to 10%' suppression, are not supported by the actual numerical choices in Eq. (102); these are benchmark values, and this should be disclosed. There is also a sign inconsistency in the O_SR discussion that must be fixed. The A_B^{μ/e} probe claim needs clearer logic. These are fixable within the manuscript's scope, so I recommend major revision rather than rejection."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Colleague,\n\nRead the Lambda_c -> (Lambda,n) semileptonic paper. The SM part is worth your time; the NP part is a benchmark study that the abstract sells as something stronger.\n\nWhat's actually new and good: the paper gives the full helicity-amplitude decomposition for all four-fermion operators, including all interferences and the tensor operator for the neutron mode. That is a genuinely complete calculation, and it's cleanly presented. The SM branching fractions come straight from Meinel's lattice form factors and are parameter-free given that input. The central values are ~10% above the BESIII measurements, which is an interesting tension to watch as both sides improve. The ratio of forward-backward asymmetries A_mu/e_FB does look robust against form-factor uncertainties, and that's a useful observable to have on the books.\n\nNow the soft spots. The NP analysis in Sec. 3.3 is not what the abstract claims. The Wilson coefficients in Eq. (102) - C_SL=C_SR=0.3, C_VL=0.03 - are not the fitted values from D/D_s decays. The text itself says the fits give C_V,R ~1e-3 and C_S ~1e-2, and Ref. [56] allows scalars at most ~0.1. The authors then pick 0.3 to 'maximize the manifestation.' That's fine as an illustrative benchmark, but the abstract says the coefficients were 'fitted from D and D_s meson decays,' and the headline 'up to 10%' suppression is linear in the scalar coefficient. With a realistic fitted value the effect would shrink to a few percent and disappear inside the form-factor uncertainties. This needs to be stated clearly.\n\nThere's also a sign inconsistency: Sec. 3.3 says the SM overestimate favors a negative C_SR, yet Eq. (102) and the figures use positive C_SR=0.3 and claim O_SR suppresses the rate. Either the sign convention is off or the statement is. That should be resolved.\n\nMinor: the claim that A_mu/e_FB is a 'novel probe for NP' is oversold. If it's largely unaffected by the NP operators considered, then a deviation would point to something else - maybe form factors or SM input. It's a good null-test observable, not a NP discoverer.\n\nOverall: the formal work is solid and the SM predictions are useful. The NP section needs honesty about what is fitted vs benchmark, and the abstract needs to match. This deserves a serious referee; with revisions it would be a solid contribution to the charm semileptonic literature.","headline":"Solid SM calculation with a complete helicity formalism; the NP section is a benchmark study the abstract overstates as a fit-based prediction.","tokens_in":30853,"tokens_out":5264,"would_cite":true,"duration_ms":48900,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":["12.15.Hh","13.30.Ce","14.20.Lq"],"model":"deepseek-v4-flash","headline":"Standard Model Λc decays run about 10 percent above experiment, and a muon-only scalar operator could close the gap.","keywords":["Λc semileptonic decays","charm baryon","effective Hamiltonian","helicity amplitudes","new physics","lepton flavor universality","lattice QCD form factors","forward-backward asymmetry"],"falsifier":"A measurement of B(Λc → ne+νe) and B(Λc → nμ+νμ) at the 5-10 percent level, combined with an improved measurement of the Λ modes, would directly test the size and sign of the claimed 10 percent offset; if the neutron-mode branching fractions land at the lattice value near 4.1 × 10⁻³ while the Λ modes land at the current central values, the pattern predicted here would not hold. A precise measurement of the ratio A_μ/e_FB in the neutron channel, where tensor form factors are available, would similarly confirm or rule out the claim that this ratio is unaffected by the fitted new physics operators.","tokens_in":29832,"feed_emoji":"⚛️","tokens_out":6501,"duration_ms":56146,"temperature":0.7,"pith_summary":"This paper studies the weak decays of the charmed baryon Λc into a Λ baryon or a neutron plus a charged lepton and neutrino, using lattice QCD form factors and a model-independent effective Hamiltonian that includes every possible four-fermion operator. The authors find that the Standard Model branching fractions for both channels are approximately 10 percent larger than the current experimental central values, and they examine whether new physics coupling only to muons could explain or alter this pattern. Their key concrete results are that a right-handed scalar operator with Wilson coefficient C_SR = 0.3 can suppress the muonic Λc → Λ and Λc → n branching fractions by up to 10 percent, and that the ratio of forward-backward asymmetries between muon and electron modes is a robust probe because hadronic uncertainties largely cancel in it. The paper also provides a complete set of helicity amplitudes and differential observables that can be compared directly with data from ongoing charmed-baryon experiments.","feed_headline":"A clean ratio probe for new physics in Λc decays","feed_subtitle":"SM branching fractions run 10 percent above experiment; a muon-only scalar operator could explain the gap.","key_machinery":"The central machinery is the helicity-amplitude decomposition of the three-body decay Λc → (Λ,n)ℓν, built from lattice QCD form factors expanded in the z variable with pole factors defined in Eq. (11). The paper constructs the complete hadronic helicity amplitudes for vector, axial-vector, scalar, pseudoscalar, and tensor operators, matches them to the lepton-side helicity amplitudes, and builds the full differential decay distribution including the interference terms between all pairs of operators. The ratio A_μ/e_FB(q²) is defined in Eq. (101) as the ratio of the muon-mode to electron-mode forward-backward asymmetries, and the claim is that the form-factor uncertainties cancel in this double ratio because numerator and denominator depend on the same hadronic amplitudes.","core_discovery":"The paper claims that, using the lattice QCD form factors of Refs. [21] and [45] and the full set of c → (s,d)ℓν effective operators, the Standard Model branching fractions are B(Λc → Λμ+νμ) = (3.75 ± 0.19)%, B(Λc → Λe+νe) = (3.88 ± 0.19)%, B(Λc → nμ+νμ) = (4.05 ± 0.29) × 10⁻³, and B(Λc → ne+νe) = (4.15 ± 0.29) × 10⁻³. These central values sit roughly 10 percent above the experimental measurements of the Λ modes, which are (3.48 ± 0.20)% and (3.56 ± 0.13)%, respectively, although the central values agree within combined uncertainties. For new physics that couples only to the muon, with Wilson coefficients fitted from D and Ds decays, the paper finds that the right-handed scalar operator O_SR, with magnitude 0.3, suppresses the muonic branching fractions by up to 10 percent, and it notes that the sign of the coefficient is negative if the experimental deficit reflects a real new-physics effect. The left-handed vector operator enhances the rates by a similar amount. The paper further claims that the ratio A_μ/e_FB of the muon to electron forward-backward asymmetries is essentially insensitive to the fitted new physics operators and to hadronic form-factor uncertainties, making it a distinctive observable whose measured value should track the Standard Model prediction if no new physics affects this decay.","pith_inferences":["If the same 10 percent suppression pattern appears in both the strangeness-changing and non-strangeness-changing modes, a common new-physics interpretation would need an operator that acts on both c → s and c → d transitions with comparable strength, which is a testable pattern that the paper's single-operator analysis does not yet combine.","The paper's A_μ/e_FB ratio could be sharpened by constructing a fully integrated double ratio or a weighted q² average, which would produce a single number with even smaller reported uncertainty and a cleaner experimental target.","A future measurement of the neutron-mode branching fractions with 5-percent precision would discriminate between the lattice QCD prediction (around 4.1 × 10⁻³) and the lower quark-model predictions (near 3 × 10⁻³), independently of the new physics question.","The absence of tensor form factors for the Λc → Λ transition means the tensor operator's effects are only partially mapped; lattice calculations of those form factors would complete the new-physics reach of this observable set."],"forward_implications":["If the 10 percent offset between the Standard Model central values and the experimental measurements persists with smaller errors, it would favor a destructive new-physics contribution in the muon channel of the size explored here, rather than a pure Standard Model description.","The first observation of Λc → ne+νe, reported after this analysis was completed, gives (3.57 ± 0.34 ± 0.14) × 10⁻³, which is consistent with the paper's prediction and shows the same slight deficit that the Λ modes show, so a common shift in the c → s and c → d channels is worth testing.","A precise measurement of the ratio A_μ/e_FB in either the Λ or neutron mode would provide a nearly form-factor-independent test of lepton flavor universality that is complementary to the branching-fraction ratio R_μ/e.","The complete set of helicity amplitudes, including all operator interferences, can be reused directly in any future analysis that assumes more than one new physics operator at a time.","The tensor operator's effect on the convexity parameter in the Λc → nμ+νμ channel provides a way to distinguish a tensor contribution from the scalar operators, which affect the polarization observables instead."],"supporting_citations":[{"why":"Supplies the lattice QCD z-expansion form factors for Λc → Λ, the main numerical input for all Λ-mode branching fractions and observables.","marker":"[21]"},{"why":"Supplies the lattice QCD form factors, including tensor form factors, for Λc → n, the main numerical input for the neutron-mode results.","marker":"[45]"},{"why":"Provides the experimental measurement of B(Λc → Λe+νe) = (3.56 ± 0.11 ± 0.07)%, the baseline the paper compares against.","marker":"[26]"},{"why":"Provides the experimental measurement of B(Λc → Λμ+νμ) and the ratio R_μ/e, the baseline for the muon-mode comparison.","marker":"[27]"},{"why":"The authors' previous minimum χ² fit of Wilson coefficients from D and Ds decays, which sets the new physics coefficient ranges adopted in this paper.","marker":"[18]"},{"why":"Defines the effective Hamiltonian with the full set of four-fermion operators that the paper's model-independent framework is built on.","marker":"[15]"},{"why":"Supplies the alternative Wilson coefficient ranges (C_V up to 10⁻², C_S up to 10⁻¹) used to set the adopted new physics benchmark values.","marker":"[56]"},{"why":"Reports the first observation of Λc → ne+νe, which the paper compares against its Standard Model prediction in the note added.","marker":"[57]"}],"fun_headline_variants":["Λc decays: SM overshoots experiment by 10%","Right-handed scalar could cut Λc muon rates","Forward-backward ratio in Λc probes new physics","Λc: a clean ratio to test new physics","SM vs experiment in Λc: scalar to the rescue?"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The lattice QCD form factors from Refs. [21] and [45], including their z-expansion coefficients and quoted uncertainties, are correct; every numerical prediction in the paper inherits that external input, and if the form factors are off by more than their stated errors the 10 percent tension with experiment could disappear or change sign.","fun_headline_variants_meta":{"raw":{"variants":["Λc decays: SM overshoots experiment by 10%","Right-handed scalar could cut Λc muon rates","Forward-backward ratio in Λc probes new physics","Λc: a clean ratio to test new physics","SM vs experiment in Λc: scalar to the rescue?"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000237,"raw_usage":{"total_tokens":1675,"prompt_tokens":1283,"completion_tokens":392,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":899,"completion_tokens_details":{"reasoning_tokens":312}},"tokens_in":899,"tokens_out":392,"duration_ms":4119,"temperature":1.0,"reasoning_tokens":312,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-10T16:42:59.507350+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"A measurement of B(Λc → ne+νe) and B(Λc → nμ+νμ) at the 5-10 percent level, combined with an improved measurement of the Λ modes, would directly test the size and sign of the claimed 10 percent offset; if the neutron-mode branching fractions land at the lattice value near 4.1 × 10⁻³ while the Λ modes land at the current central values, the pattern predicted here would not hold. A precise measurement of the ratio A_μ/e_FB in the neutron channel, where tensor form factors are available, would similarly confirm or rule out the claim that this ratio is unaffected by the fitted new physics operators.","supporting_citations":[{"cited_title":"Ablikim et al., Study of the Semileptonic Decay Λ + c → Λ e+νe, Phys","cited_arxiv_id":null,"evidence_quote":"Provides the experimental measurement of B(Λc → Λe+νe) = (3.56 ± 0.11 ± 0.07)%, the baseline the paper compares against."}],"review_version":1}