Semi-leptonic decays B to D^((*))(1S,2S)ell ν_(ell) within the covariant light-front approach
Pith reviewed 2026-06-26 20:49 UTC · model grok-4.3
The pith
Covariant light-front quark model calculations of B to D(*) semi-leptonic decays reproduce the R(D) and R(D*) anomalies at 3.1 and 2.1 sigma from HFLAV averages.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
Within the covariant light-front quark model the calculated ratios R(D) and R(D*) lie 3.1 sigma and 2.1 sigma below the current HFLAV world averages while lying closer to the latest LHCb measurements; the model also produces branching fractions for the radially excited 2S channels that are larger than Bethe-Salpeter results but consistent with relativistic quark model estimates.
What carries the argument
Covariant light-front quark model transition form factors for B to D(*) (1S,2S) that are inserted into the semi-leptonic width formulas to obtain branching ratios and angular observables.
If this is right
- Branching ratios for B(s) to D(s)(2S) and D*(s)(2S) semi-leptonic decays fall between 10^{-4} and 10^{-3}.
- Forward-backward asymmetries and longitudinal polarization fractions for all modes are consistent with most other theoretical calculations and available data.
- The tau-mode deficit relative to light-lepton modes appears automatically from the form-factor ratios without additional parameters.
- Predictions for the excited-state channels can be tested once those modes are observed.
Where Pith is reading between the lines
- If the model's form factors remain reliable at higher recoil, the same framework can be applied to related decays such as B to D** or Bs to Ds** without retuning.
- Discrepancy with HFLAV but agreement with LHCb suggests the anomaly may be sensitive to the precise experimental averaging procedure.
- The 2S branching ratios being larger than Bethe-Salpeter but matching relativistic quark model points to a systematic difference between light-front and Bethe-Salpeter treatments of radial excitations.
Load-bearing premise
The covariant light-front quark model supplies accurate form factors for every B to D(*) (1S and 2S) transition and every lepton flavor.
What would settle it
A future measurement of R(D) that lies more than two standard deviations away from 0.261 would contradict the central numerical prediction obtained from the model's form factors.
Figures
read the original abstract
We present a systematic analysis of the semi-leptonic decays $B_{(s)}\to D_{(s)}(1S,2S)\ell\nu_\ell$ and $B_{(s)}\to D^*_{(s)}(1S,2S)\ell\nu_\ell$ with $\ell=e,\mu,\tau$ within the covariant light-front quark model (CLFQM). Using the form factors of the transitions $B_{(s)}\to D_{(s)}(1S,2S)$ and $B_{(s)}\to D^*_{(s)}(1S,2S)$, we calculate the branching ratios of the relevant semi-leptonic decays and find that $Br(B_{(s)}\to D_{(s)}\ell^\prime\nu_{\ell^\prime})$ and $Br(B_{(s)}\to D^*_{(s)}\ell^\prime\nu_{\ell^\prime})$ with $\ell^\prime=e,\mu$ are agree well with the data, while $Br(B_{(s)}\to D_{(s)}\tau\nu_{\tau})$ and $Br(B_{(s)}\to D^*_{(s)}\tau\nu_{\tau})$ are systematically smaller than the experimental measurements. This naturally gives rise to the so-called $\mathcal{R}(D)$ and $\mathcal{R}(D^*)$ anomalies. Our predictions $\mathcal{R}(D)=0.261\pm0.013$ and $\mathcal{R}(D^*)=0.228\pm0.026$ show $3.1\sigma$ and $2.1\sigma$ deviations from the current experimental world averages compiled by the Heavy Flavor Averaging Group (HFLAV), respectively, yet only deviate by $0.16\sigma$ and $1.5\sigma$ from the latest LHCb measurements. For the decays $B_{(s)}\to D_{(s)}(2S)\ell\nu_\ell$ and $B_{(s)}\to D^*_{(s)}(2S)\ell\nu_\ell$, their branching ratios lie in the range $10^{-4}\sim10^{-3}$, which are much larger than the results from the Bethe Salpeter (BS) equation , but agree with the relativistic quark model (RQM) calculations. Furthermore, we also calculate the forward-backward asymmetries $\mathcal{A}_{FB}$ and longitudinal polarization fractions $f_L$ for the corresponding decays. Our predictions are consistent with most other theoretical results and experimental data
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper performs a systematic calculation of branching ratios for the semi-leptonic decays B_{(s)} o D_{(s)}^{( * )}(1S,2S) \ell u_\ell (\ell = e, u, au) in the covariant light-front quark model. Form factors are computed from Gaussian wave functions whose parameters are fixed by meson masses and decay constants; these are inserted into the standard differential decay rates to obtain Br values. Light-lepton modes are reported to agree with data while au modes are smaller, yielding R(D) = 0.261 \pm 0.013 (3.1 \sigma from HFLAV) and R(D*) = 0.228 \pm 0.026 (2.1 \sigma from HFLAV). Predictions for the 2S states lie in the 10^{-4}--10^{-3} range, and A_{FB}, f_L are also computed.
Significance. If the CLFQM form-factor shapes are reliable across the full q^2 range, the quoted R(D(*)) values would constitute an independent indication of the anomalies and supply useful benchmarks for the excited-state channels. The reported consistency with light-lepton data and with selected other models is a positive feature, but the overall significance is reduced by the absence of direct external validation of the q^2 dependence.
major comments (2)
- [Form-factor and numerical-results sections] The central R(D) and R(D*) results rest on the assumption that the CLFQM form factors (f_+, f_0, V, A_0, A_1, A_2) correctly describe the high-q^2 region probed by the au modes. No comparison is presented to lattice-QCD determinations (HPQCD, FNAL/MILC, JLQCD) at q^2 = 0 or at w = 1, nor is the slope or curvature of the form factors tested against lattice data; this validation is load-bearing for the claimed 3.1 \sigma and 2.1 \sigma deviations.
- [Methodology and results sections] The same set of constituent-quark masses and Gaussian eta parameters controls both the overall normalization (fixed by light-lepton rates) and the entire q^2 shape. Consequently, agreement with Br(B o D^{(*)} e/ u) does not furnish an independent constraint on the high-q^2 integrals that dominate the au rates; this circularity is not quantified.
minor comments (3)
- [Abstract] Abstract contains a grammatical error: 'are agree well with the data' should read 'agree well with the data'.
- [Abstract and results section] The abstract states that the 2S branching ratios 'agree with the relativistic quark model (RQM) calculations' but does not cite the specific RQM references or tabulate the numerical comparison.
- [Abstract] Notation for the lepton flavors (\ell' vs au) and the inclusion of B_s, D_s channels could be clarified in the abstract for immediate readability.
Simulated Author's Rebuttal
We thank the referee for the careful reading of our manuscript and the constructive comments. We address the two major comments point by point below, indicating planned revisions where appropriate.
read point-by-point responses
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Referee: [Form-factor and numerical-results sections] The central R(D) and R(D*) results rest on the assumption that the CLFQM form factors (f_+, f_0, V, A_0, A_1, A_2) correctly describe the high-q^2 region probed by the τ modes. No comparison is presented to lattice-QCD determinations (HPQCD, FNAL/MILC, JLQCD) at q^2 = 0 or at w = 1, nor is the slope or curvature of the form factors tested against lattice data; this validation is load-bearing for the claimed 3.1 σ and 2.1 σ deviations.
Authors: We agree that direct comparisons of our form factors to lattice QCD results at q^2=0, w=1, and for the slope/curvature would strengthen validation of the high-q^2 behavior relevant to tau modes. Our parameters are fixed from masses and decay constants independently of lattice data, but we acknowledge the value of such benchmarks. In the revised manuscript we will add explicit comparisons at these kinematic points to available lattice results from HPQCD, FNAL/MILC and JLQCD, together with a brief discussion of any differences in q^2 dependence. revision: yes
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Referee: [Methodology and results sections] The same set of constituent-quark masses and Gaussian β parameters controls both the overall normalization (fixed by light-lepton rates) and the entire q^2 shape. Consequently, agreement with Br(B → D(*) e/μ) does not furnish an independent constraint on the high-q^2 integrals that dominate the τ rates; this circularity is not quantified.
Authors: We clarify that the constituent quark masses and Gaussian β parameters are fixed exclusively by the meson masses and decay constants, as stated in the manuscript; they are not adjusted to the light-lepton branching ratios. The light-lepton Br values are therefore genuine predictions of the model, providing an independent test of the form-factor shapes across the full kinematic range. We will revise the methodology and results sections to emphasize this distinction and to discuss more explicitly how the parameter choice affects the high-q^2 integrals relevant to the tau modes. revision: yes
Circularity Check
No circularity: CLFQM form-factor computation and R(D) predictions form an independent derivation chain
full rationale
The paper computes transition form factors in the covariant light-front quark model from meson wave functions and then inserts those form factors into standard differential decay-rate formulas to obtain branching ratios and the ratios R(D), R(D*). This is a conventional model-to-observable pipeline; the inputs (quark masses, Gaussian parameters β fitted to masses and decay constants) are distinct from the output ratios, and no equation reduces the R values to the fit inputs by algebraic identity. No self-citation is invoked as a uniqueness theorem, no ansatz is smuggled, and the light-lepton agreement is presented as an a-posteriori check rather than a fitted constraint that forces the tau-mode result. The derivation therefore remains self-contained against external benchmarks.
Axiom & Free-Parameter Ledger
free parameters (1)
- CLFQM constituent quark masses and wave-function parameters
axioms (1)
- domain assumption The covariant light-front quark model yields reliable form factors for B to D(*) (1S,2S) transitions at all momentum transfers relevant to semi-leptonic decays.
Reference graph
Works this paper leans on
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013 and R(D∗) = 0 . 228 ± 0. 026 show 3 . 1σ and 2 . 1σ deviations from the current experimental world averages compiled by the Heavy Flavor Averaging Group (HFLA V), respectively, yet only deviate by 0 . 16σ and 1. 5σ from the latest LHCb measurements. For the decays B(s) → D(s)(2S)ℓνℓ and B(s) → D∗ (s)(2S)ℓνℓ, their branching ratios lie in the range 10 ...
Pith/arXiv arXiv 2026
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For the general P → P transition, the amplitude for the lowest order is AP P µ = − i3 Nc (2π)4 ∫ d4p′ 1 H ′ P H ′′ P N ′ 1N ′′ 1 N2 SP P µ , (11) 4 where N ′(′′) 1 = p′(′′)2 1 − m′(′′)2 1 and N2 = p2 2 − m2 2 arise from the quark propagators, and the trace SP P µ can be obtained directly by using the Lorentz contraction, SP P µ = Tr [ γ5 (̸ p′′ 1 + m′′
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γ5 (− ̸ p2 + m2)] , (12) where the analytical expression for SP P µ can be found in Ref. [23]. It is similar for the P → V transition amplitude, AP V µ = − i3 Nc (2π)4 ∫ d4p′ 1 H ′ P (iH ′′ V ) N ′ 1N ′′ 1 N2 SP V µν ε∗ν, (13) where SP V µν = ( SP V V − SP V A ) µν = Tr [( γν − 1 W ′′ V (p′′ 1 − p2)ν ) (p′′ 1 + m′′
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(γµ − γµ γ5) (̸ p′ 1 + m′
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(14) In practice, we use the light-front decomposition of the Feynman lo op momentum and integrate out the minus component through the contour method
γ5 (− ̸ p2 + m2) ] . (14) In practice, we use the light-front decomposition of the Feynman lo op momentum and integrate out the minus component through the contour method. If the covariant vertex functions are not singular when performing integration, the trans ition amplitudes will pick up the singularities in the anti-quark propagators. The integration ...
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5 × 10− 2
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249 ± 0. 043 ± 0. 047 given by LHCb, the RD measurements exhibit a decreasing trend over time. Accordingly, relative to the experimental measurement s, our prediction R (D) =
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014 yields a shrinking tension, which changes from 2
260± 0. 014 yields a shrinking tension, which changes from 2 . 4σ to 0. 16σ, while the deviation compared to the HF AG value still amounts to 3. 1σ. It is similar for the R(D∗) values between our predictions and the data. For example, our value R(D∗− ) = 0 . 228± 0. 026 compares with the measurement 0 . 260 ± 0. 015 ± 0. 016 ± 0. 012 given by LHCb [45], c...
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1σ and 2. 1σ, respectively. Meanwhile, the branching ratios of the decays B → D(∗)(2S)ℓνℓ are also calculated, which lie in the range 10 − 4 to 10 − 3 and are about one order of magni- tude larger than those obtained from the BS equation. For decays Bs → D(∗) s (2S)ℓνℓ, our predictions are 3 to 5 times those given by the BS equation, and are c onsistent w...
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discussion (0)
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