{"id":"b891afa4-dc5f-460e-bb3f-bffae9a4c0e5","arxiv_id":"1908.08967","paper_version":2,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":4.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":1,"one_line_summary":"In the B -> D* tau neutrino decay, the longitudinal polarization fraction F_L barely changes with the V-A breaking parameter alpha, while the difference between the two transverse polarization components is highly sensitive to alpha.","lead":"This paper calculates how the polarization of the D* meson in the decay B -> D* tau neutrino changes if the weak quark current is modified from the standard V-A form. It finds that the longitudinal polarization stays nearly constant while the difference between the two transverse components is much more sensitive, suggesting that transverse polarization is a better probe for new physics.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The unquantified cancellation of intrinsic quark form factors is the load-bearing assumption; the transverse-difference sensitivity is an interference term where imperfect cancellation would change the conclusion.","rationale":"Read in good faith, the paper makes a narrow, clearly stated claim: within a one-parameter family of b to c currents gamma^mu - alpha gamma^mu gamma^5, computed in the quark model of Ref. [30], F_L is only weakly dependent on alpha while the normalized difference of the two transverse D* helicity contributions is strongly dependent. Equations (5) are the entire basis for this ordering. The SM check (alpha=1) gives F_L=0.456, close to the value quoted from Ref. [28], which supports the model at the level of one integrated ratio. However, the paper explicitly concedes in Section I that intrinsic quark form factors are neglected and merely 'claimed to approximately cancel in the ratios.' The central observables are all ratios, so the cancellation is not optional; it is doing work. The transverse-difference observable is an interference between an alpha-weighted axial piece and an alpha-independent vector piece, so its sensitivity to alpha depends on the relative q^2 evolution of the vector and axial form factors. Intrinsic form factors would generically introduce different q^2 slopes for V and A, and that is exactly the input that changes the interference. The agreement on F_L at alpha=1 is not a sharp test of this, because F_L at alpha=1 is dominated by axial contributions and is weakly sensitive to the vector-axial interference; the very observable proposed as a null-test of BSM is the most exposed to the omitted physics. Thus the reader's conditional verdict is appropriate: the central claim is plausible and internally consistent, but the load-bearing assumption is unquantified. I recommend no change to the verdict, pending a form-factor-robustness check.","tokens_in":5538,"tokens_out":15638,"duration_ms":158324,"concrete_test":"Recompute the α-scans of Table I and Fig. 5 with the same quark current gamma^mu - alpha gamma^mu gamma^5 but standard B to D* form factors (e.g., the CLN parameterization or lattice results for V, A0, A1, A2), replacing the point-like overlap factors in Eq. (5). If the normalized M'=-1 minus M'=+1 difference remains the most α-sensitive observable and the F_L variation over alpha in [0.8,1.2] stays below the Belle uncertainty, the concern is resolved; if the sensitivity ordering changes or the difference becomes comparable to the F_L variation, the central claim fails.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The paper's observable predictions are ratios: F_L and the normalized transverse components in Figs. 4-5 are obtained from Eq. (5) after dividing by the total rate. The quark-model expressions in Eq. (5) treat the b and c quarks as point-like; the model of Ref. [30] omits intrinsic quark form factors, and Section I states that these are 'claimed to approximately cancel in the ratios.' This cancellation is never demonstrated or bounded. It is not a harmless overall factor: the transverse amplitudes are superpositions of the axial term (1-BB'p^2)α and the vector-like term (Bp-B'p), so the normalized difference between M'=-1 and M'=+1 is driven by the interference term 4α(1-BB'p^2)(Bp-B'p)/R. A momentum-dependent mismatch between vector and axial form factors, of the type intrinsic form factors would produce, enters precisely this interference and can alter the α-sensitivity without shifting F_L much. Since the central claim is the α-sensitivity ordering (F_L flat, transverse difference steep), the conclusion rests on an unvalidated cancellation. No internal inconsistency is apparent; the SM value F_L=0.456 matching Ref. [28] is an encouraging check, but it does not constrain the form-factor dependence of the α-sensitive interference.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper studies the helicity amplitudes of the decay Bbar -> D* anti-nu_tau tau using a quark-level current of the form gamma^mu(1 - alpha gamma^5). For alpha = 1 the standard-model V-A case is recovered, and for alpha != 1 the model represents a family of V-A breaking scenarios. The authors compute the longitudinal polarization fraction F_L^{D*} and the normalized transverse M' = -1 and M' = +1 contributions, integrating the differential rates over the invariant mass of the nu-tau tau pair. Their main finding is that F_L^{D*} is nearly independent of alpha, while the difference between the two transverse helicity contributions is strongly alpha-dependent. On this basis they recommend measuring the transverse helicity components as a more sensitive probe of beyond-standard-model physics than the longitudinal component. The calculation is presented as an extension of the authors' earlier quark-model treatment of B -> D* anti-nu_l l, and the paper emphasizes that no free parameters are fitted.","tokens_in":5756,"tokens_out":6348,"duration_ms":59642,"significance":"If the central claim survives scrutiny, the paper provides a useful and falsifiable phenomenological message: for a broad class of modified quark currents, the longitudinal polarization is a poor discriminator, whereas the M' = -1 versus M' = +1 transverse asymmetry is a sensitive probe. The calculation has the virtue of being parameter-free in the sense that alpha is scanned rather than fitted, and the SM point F_L = 0.456 agrees well with independent SM calculations. However, the significance is presently limited by the model-dependence of the hadronic matrix elements and, in particular, by the unquantified cancellation of intrinsic quark form factors on which the ratio predictions rest. The paper would become substantially stronger if the model were validated against the Belle measurement that motivates it and if the form-factor cancellation were demonstrated rather than asserted.","major_comments":[{"comment":"Section I states that the model of Ref. [30] 'neglects the contribution of intrinsic quark form factors, which are claimed to approximately cancel in the ratios evaluated there.' This is an assertion, not a demonstrated property, and it is load-bearing because the predictions in Figs. 4 and 5 are normalized ratios built from Eq. (5). The M' = +1 and M' = -1 amplitudes are superpositions of (1 - BB'p^2)alpha and (Bp - B'p), so the normalized difference is governed by the interference term 4 alpha (1 - BB'p^2)(Bp - B'p)/R. Momentum-dependent corrections that do not factorize would enter this interference directly and could alter the alpha-sensitivity ordering even if F_L remained close to 0.456. Please either demonstrate the cancellation quantitatively, for example by including a simple dipole form factor and showing that the ratios are stable, or restrict the claims to the specific model without the cancellation assumption.","section":"Section I and Eq. (5)"},{"comment":"The paper is motivated by the Belle measurement F_L^{D*} = 0.60 +/- 0.08 +/- 0.04 quoted in Eq. (1), but it never quantitatively compares the model predictions with that measurement. In Table I, F_L ranges only from 0.415 to 0.465 for alpha between 0.5 and 1.5, so the central value is below the Belle measurement by about 1.6 sigma and no model point reaches 0.60. The statement that F_L is not a good BSM probe therefore requires qualification: within this family the observable is not only insensitive to alpha, it is also in tension with the very measurement that motivates the paper. A fit or at least a quantitative discussion of this discrepancy is needed before drawing the conclusion that experimental effort should be redirected to the transverse components.","section":"Section III, Table I"},{"comment":"The central formulas are introduced with 'we find' and no derivation is shown in the paper. The text refers to the approach of Ref. [30] but does not show how the quark current of Eq. (2) is mapped onto the meson-level amplitudes, how the tau mass is handled beyond the kinematical factors of Eqs. (3)-(4), or how the interference terms in the M' = +/-1 amplitudes arise. Since all quantitative claims and figures follow from Eq. (5), please provide the derivation in the text or give a precise equation-by-equation mapping to Refs. [30] and [31] so that the extension from the massless-lepton case to the nu_tau tau case can be checked.","section":"Section II, Eqs. (5-a)-(5-c)"}],"minor_comments":[{"comment":"The column header uses f_D*_L while the text uses F_L^{D*}; the notation should be unified.","section":"Table I"},{"comment":"Equation (9) is used twice, once for the total differential rate R and once for the numerical value F_L^{D*} = 0.456; renumber to avoid confusion.","section":"Section III"},{"comment":"References [22] and [24] are the same paper (Tanaka and Watanabe, Phys. Rev. D 87, 034028 (2013)); the duplicate should be removed or replaced with a different intended reference.","section":"References"},{"comment":"The abstract and Section III write M = -1 and M = +1 without the prime that denotes the D* helicity in Section II; use M' consistently.","section":"Abstract and Section III"},{"comment":"The phrase 'right handed quark currents' is imprecise for the current gamma^mu(1 - alpha gamma^5): changing alpha changes the axial-vector coupling, and for negative alpha the structure is closer to V + A; rephrase to avoid confusion.","section":"Summary"}],"recommendation":"major_revision","confidential_remarks":"For the editor: the manuscript is very short and its main quantitative step is delegated to two earlier papers by the same authors. I would encourage requesting the derivation itself, or a supplementary appendix, as part of the revision; the issue is reproducibility of the central equations, not a question of authorship. The Belle comparison in Section III may also need a quantitative treatment before the paper's phenomenological recommendation can be fully assessed."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Short version: the paper makes a timely and concrete point — for the V−A-breaking current gamma^mu − alpha gamma^mu gamma_5, F_L in B->D* tau nu is nearly flat in alpha while the normalized M'=−1 minus M'=+1 difference is the sensitive observable. If I were planning Belle II angular measurements, I would want to know this. The quantitative case is weaker than the abstract suggests, but the paper deserves a serious referee.\n\nWhat is actually new: the tau-mode calculation with lepton mass effects extends the authors' earlier light-lepton study [30], and it is the first application of that model to the Belle F_L measurement. The SM check is clean — they get F_L=0.456, matching Ref. [28]'s 0.457±0.010 — and the figures make the ordering among helicity components obvious. The model has no fitted free parameters. That is real credit, and the citation pattern is fine: [30] is the source of the formalism and the SM benchmarks are external.\n\nThe soft spots are real but not disqualifying. Equations (5) are quoted from [30] without derivation, so a referee cannot check how the quark-model spinors map onto meson states. More importantly, the paper states that intrinsic quark form factors 'approximately cancel in the ratios' but never demonstrates or bounds that cancellation. This matters because the sensitive observable is the interference term 4 alpha (1−BB'p^2)(Bp−B'p)/R. An overall factor would cancel, but a momentum-dependent mismatch between vector and axial vertices enters precisely that term and can change the alpha-sensitivity without moving F_L much. The stress-test note lands: this is the load-bearing assumption.\n\nTwo smaller issues. The model's F_L values run from 0.415 to 0.465, all below Belle's central 0.60; the paper never mentions the tension, even though it is only roughly 1.6 sigma. And the predictions have no systematic error bars, so the 'bands' in Fig. 5 are not uncertainty bands. Minor: the phase-space prefactor in Eq. (3) looks dimensionally odd if m_nu is the neutrino mass; it cancels in the ratios plotted, but a referee should ask for the definition.\n\nBottom line: a useful, clear phenomenological note for experimentalists and for theorists choosing angular observables. I would send it to peer review, not desk-reject it, but I would ask for a revised version that either derives the amplitudes or delimits the form-factor cancellation, and that compares with the Belle measurement instead of just citing it.","headline":"Useful, timely suggestion to measure transverse D* helicities in B->D* tau nu; the quantitative case rests on an unquantified form-factor cancellation and ignores the Belle central-value tension.","tokens_in":6291,"tokens_out":7226,"would_cite":true,"duration_ms":77590,"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":"In B→D*τν, the transverse helicity difference, not F_L, probes V−A breaking.","keywords":["B→D*τν decay","D* longitudinal polarization","helicity amplitudes","V−A breaking","right-handed quark currents","b→c l ν transition","beyond Standard Model"],"falsifier":"A measurement of the normalized $M'=+1$ and $M'=-1$ contributions in $\\bar B \\to D^* \\bar\\nu_\\tau \\tau$ as a function of $M^{(\\nu\\tau)}_{\\rm inv}$ that showed their difference staying flat across the spectrum, or lying far outside the band the model predicts for $\\alpha\\in[0.8,1.2]$ while $F_L^{D^*}$ varies as predicted, would falsify the claim that this difference is the sensitive probe.","tokens_in":5308,"feed_emoji":"","tokens_out":7706,"duration_ms":75540,"temperature":0.7,"pith_summary":"The paper asks whether the recently measured longitudinal polarization fraction $F_L^{D^*}$ in $\\bar B \\to D^* \\bar\\nu_\\tau \\tau$ can expose quark currents beyond the Standard Model. Working with the family of quark currents $\\gamma^\\mu(1-\\alpha\\gamma_5)$, where $\\alpha=1$ is the Standard Model, the authors compute all three $D^*$ helicity contributions. They find $F_L^{D^*}$ moves only from about 0.415 to 0.465 as $\\alpha$ runs from $-0.5$ to $1.5$, so for this family the longitudinal fraction is a poor discriminator. The two transverse components, $M'=-1$ and $M'=+1$, change much more strongly, and their difference is the most sensitive observable, even flipping sign when $\\alpha$ flips sign. The paper therefore urges experimental study of the transverse helicity components.","feed_headline":"Transverse helicity, not F_L, reveals V−A breaking in D* decays","feed_subtitle":"In B→D*τν, α shifts the longitudinal fraction barely but flips the M'=±1 difference sign.","key_machinery":"The central object is the generalized quark transition current $Q_\\mu = \\langle \\bar u_c | \\gamma_\\mu(1-\\alpha\\gamma_5) | u_b \\rangle$, with $\\alpha=1$ recovering the Standard Model $V-A$ current. Around it the paper builds helicity amplitudes for the three $D^*$ spin projections $M'=0,\\pm1$, evaluated in the $\\bar\\nu_\\tau\\tau$ rest frame, using a quark-model mapping of quark momenta to meson momenta consistent with heavy-quark symmetry. Equations (5a)--(5c) give the summed squared amplitudes for each $M'$ in terms of meson wave-function factors $A,B,A',B'$ and the momentum $p$; the sensitive observable is the difference between $M'=-1$ and $M'=+1$, whose expressions carry the $(Bp-B'p)$ term with opposite signs, so the difference changes sign when $\\alpha$ changes sign.","core_discovery":"On its own terms, the paper establishes that in the quark-model treatment of Ref. [30] extended to $\\bar B \\to D^* \\bar\\nu_\\tau \\tau$, the longitudinal polarization fraction $F_L^{D^*}$ is almost flat under changes in the $V-A$ breaking parameter $\\alpha$: the computed values are 0.415, 0.448, 0.456, 0.461, 0.465 for $\\alpha=0.5,0.8,1.0,1.2,1.5$, and again 0.415 for $\\alpha=-0.5$. At $\\alpha=1$ the result 0.456 matches the most recent Standard Model prediction. The normalized $M'=0$ band over $\\alpha\\in[0.8,1.2]$ is narrow, while the normalized $M'=-1$ and $M'=+1$ curves spread widely and their difference is the quantity most sensitive to $\\alpha$. The intended conclusion is that for this family of models $F_L^{D^*}$ is not a good probe of new physics, and the difference between the two transverse helicity contributions is the recommended observable.","pith_inferences":["A natural extension is that helicity-integrated asymmetries in other $b\\to c\\tau\\nu$ decays, not only $\\bar B\\to D^*\\bar\\nu_\\tau\\tau$, may also dilute $V-A$ breaking signals, and only differential helicity or angular observables can separate the $\\alpha$ parameter cleanly.","If intrinsic quark form factors do not cancel as assumed, the absolute transverse rates could shift, but the sign-flip structure of the $M'=-1$ versus $M'=+1$ difference is driven by the opposite sign of the $(Bp-B'p)$ term and may be more robust than the size of the effect.","A testable extension would be to compare the same transverse helicity difference in $\\bar B\\to D^* \\ell \\bar\\nu$ decays for $\\ell=e,\\mu$ against the $\\tau$ mode, since right-handed current effects depend on the lepton mass and a lepton-flavor comparison would help isolate $\\alpha$."],"forward_implications":["The measured value $F_L^{D^*}=0.60\\pm0.08\\pm0.04$ cannot by itself discriminate this model family from the Standard Model, since the predicted range for $\\alpha\\in[0.5,1.5]$ is roughly 0.415 to 0.465.","Experiments that can separate the $M'=-1$ and $M'=+1$ transverse contributions will gain a much more sensitive handle on $V-A$ breaking than the longitudinal fraction alone.","A sign flip in the difference between the $M'=-1$ and $M'=+1$ contributions relative to the Standard Model would indicate a negative $\\alpha$, that is, a reversal of the right-handed current admixture.","The model reproduces the Standard Model $F_L^{D^*}$ with no fitted parameters, so its ratio predictions for the transverse components carry the same expected accuracy."],"supporting_citations":[{"why":"Supplies the helicity-amplitude formalism and the earlier finding that the M'=-1 versus M'=+1 difference is the most alpha-sensitive magnitude.","marker":"[30]"},{"why":"Provides the recently measured $F_L^{D^*}=0.60\\pm0.08\\pm0.04$ that this work tests against.","marker":"[23]"},{"why":"Gives the Standard Model prediction $F_L^{D^*}=0.457\\pm0.010$ that the alpha=1 result is compared with.","marker":"[28]"},{"why":"Supplies another recent Standard Model prediction, 0.441, in the comparison set for $F_L^{D^*}$.","marker":"[27]"},{"why":"Provides a further recent Standard Model estimate, 0.476, against which the model result is checked.","marker":"[35]"},{"why":"Supplies the quark-model evaluation of the operators used in the calculation.","marker":"[31]"},{"why":"Provides the heavy-quark symmetry foundation for mapping quark momenta to meson momenta.","marker":"[32]"},{"why":"Extends the heavy-quark symmetry relations used in the kinematic mapping.","marker":"[33]"},{"why":"Supplies the heavy-quark expansion framework that fixes the meson momentum mapping.","marker":"[34]"}],"fun_headline_variants":["F_L flat, M± difference flips: better probe in B→D*τν","Transverse spin difference beats F_L for V−A breaking signal","D* decay: watch M=-1 vs M=+1, not F_L, for new physics","In B→D*τν, F_L barely moves; transverse helicity gap is key","V−A breaking? Look at M=±1 split in B→D*τν, not F_L"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The central assumption is that intrinsic quark form factors cancel in the ratios used to predict the helicity fractions, so that the no-free-parameter quark model results are accurate; imperfect cancellation would change the predicted sensitivities.","fun_headline_variants_meta":{"raw":{"variants":["F_L flat, M± difference flips: better probe in B→D*τν","Transverse spin difference beats F_L for V−A breaking signal","D* decay: watch M=-1 vs M=+1, not F_L, for new physics","In B→D*τν, F_L barely moves; transverse helicity gap is key","V−A breaking? Look at M=±1 split in B→D*τν, not F_L"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000794,"raw_usage":{"total_tokens":3488,"prompt_tokens":930,"completion_tokens":2558,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":546,"completion_tokens_details":{"reasoning_tokens":2437}},"tokens_in":546,"tokens_out":2558,"duration_ms":18088,"temperature":1.0,"reasoning_tokens":2437,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-14T11:25:16.407123+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"A measurement of the normalized $M'=+1$ and $M'=-1$ contributions in $\\bar B \\to D^* \\bar\\nu_\\tau \\tau$ as a function of $M^{(\\nu\\tau)}_{\\rm inv}$ that showed their difference staying flat across the spectrum, or lying far outside the band the model predicts for $\\alpha\\in[0.8,1.2]$ while $F_L^{D^*}$ varies as predicted, would falsify the claim that this difference is the sensitive probe.","supporting_citations":[{"cited_title":"Huang, Y","cited_arxiv_id":null,"evidence_quote":"Supplies the helicity-amplitude formalism and the earlier finding that the M'=-1 versus M'=+1 difference is the most alpha-sensitive magnitude."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Provides the recently measured $F_L^{D^*}=0.60\\pm0.08\\pm0.04$ that this work tests against."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Gives the Standard Model prediction $F_L^{D^*}=0.457\\pm0.010$ that the alpha=1 result is compared with."},{"cited_title":"Tanaka and R","cited_arxiv_id":null,"evidence_quote":"Supplies another recent Standard Model prediction, 0.441, in the comparison set for $F_L^{D^*}$."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Provides the heavy-quark symmetry foundation for mapping quark momenta to meson momenta."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Extends the heavy-quark symmetry relations used in the kinematic mapping."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Supplies the heavy-quark expansion framework that fixes the meson momentum mapping."}],"review_version":1}