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arxiv: 1907.02257 · v2 · pith:KCNOS7NTnew · submitted 2019-07-04 · ✦ hep-ph · hep-ex

Lepton Flavor Universality tests through angular observables of overline{B}to D^((ast))elloverline{ν} decay modes

Damir Becirevic , Marco Fedele , Ivan Nisandzic , Andrey Tayduganov This is my paper

Pith reviewed 2026-05-25 09:27 UTC · model grok-4.3

classification ✦ hep-ph hep-ex
keywords B meson decayslepton flavor universalityangular observablesnew physicseffective field theoryR(D) ratiosWilson coefficients
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0 comments X

The pith

Angular observables in B to D(*) lepton neutrino decays can reveal new physics even when R(D(*)) ratios match the Standard Model.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper shows that the angular distributions in anti-B to D or D-star decays with a lepton and antineutrino provide tests of lepton flavor universality violation. It adds all possible Lorentz structures to the low-energy effective theory and fixes their Wilson coefficients by matching to the measured R(D(*)) ratios. The key result is that subsets of the angular observables can differ from Standard Model expectations even in cases where the ratios themselves become fully compatible with the Standard Model. This matters because the ratios alone do not exhaust the information needed to identify or rule out new physics in these channels.

Core claim

When the Wilson coefficients of vector, scalar, pseudoscalar, and tensor operators are determined exclusively by fitting to the experimental R(D(*)) values, the angular observables extracted from the differential decay rate still exhibit measurable deviations from their Standard Model predictions, allowing the Lorentz structure of any new physics to be distinguished even if the ratios R(D(*)) are later found to agree with the Standard Model.

What carries the argument

The full set of angular observables obtained from the four-fold differential decay distribution in the effective Hamiltonian that includes all Lorentz structures, with coefficients fixed by the R(D(*)) data.

If this is right

  • Different Lorentz structures produce distinct patterns across the angular observables.
  • Even a partial set of measured angles can separate scalar, tensor, and vector new physics scenarios.
  • The observables remain sensitive to new physics size after the ratios become Standard Model-like.
  • Comparison of data with Standard Model angular predictions supplies independent constraints on the coefficients.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Experiments at B factories could prioritize angular measurements to close loopholes left by ratio-only tests.
  • Absence of deviations in the angles would tighten bounds on allowed new physics models more than ratios alone.
  • Similar angular analyses might apply to related semileptonic modes to cross-check the same operators.

Load-bearing premise

New physics contributions to these decays are fully captured by the low-energy effective operators whose strengths are fixed only by the R(D(*)) ratios, without extra effects from higher-dimensional operators or unrelated processes.

What would settle it

A measurement of the angular observables that agrees with Standard Model predictions for every combination of Wilson coefficients that reproduces the measured R(D(*)) values within their uncertainties.

Figures

Figures reproduced from arXiv: 1907.02257 by Andrey Tayduganov, Damir Becirevic, Ivan Nisandzic, Marco Fedele.

Figure 1
Figure 1. Figure 1: Kinematics of the B → D∗ (→ Dπ)`ν decay. Another interesting observable for the study of the NP effects is the lepton polarization asymmetry defined from differential decay rates with definite lepton helicity: dΓ λ`=+1/2 dq2 = G2 F |Vcb| 2 192π 3m3 B q 2p λBD(q 2 )  1 − m2 ` q 2 2 m2 ` 2q 2  |eh + 0 | 2 + 3|eht | 2  , (13a) dΓ λ`=−1/2 dq2 = G2 F |Vcb| 2 192π 3m3 B q 2p λBD(q 2 )  1 − m2 ` q 2 2 |eh −… view at source ↗
Figure 2
Figure 2. Figure 2: AFB, Aλτ observables relevant to B → Dτν and AFB, Aλτ , RL,T , RA,B and A3−9 relevant to B → D∗ τν are displayed for various values of NP couplings and as functions of q 2 . The width of each curve comes from the theoretical uncertainties in hadronic form factors and quark masses. The benchmark gi ’s are chosen to be the best fit values, as discussed in the text. 12 [PITH_FULL_IMAGE:figures/full_fig_p013_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: The allowed values for the NP couplings in Eq.(5) as obtained from the the fit with R(D) exp and R(D∗ ) exp, and by switching one coupling gi at the time. Red stars denote the best fit values. Regarding the form factors we used those to which we refer in the text as CLN+HQET+LATT. The benchmark values, denoted by red stars in Figs. 3-4, correspond to the best fit values. We get gV = 0.20 + i 0.19 , gA = 0.… view at source ↗
Figure 4
Figure 4. Figure 4: Same as in [PITH_FULL_IMAGE:figures/full_fig_p017_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: Left panel: predicted p.d.f. for the observable R(A5), assuming a complex gV . Right panel: predicted p.d.f. for the observable R(A5), assuming that gV is equal to the best fit value reported in Eq. (45). imaginary. From the plot shown in the first panel of [PITH_FULL_IMAGE:figures/full_fig_p018_5.png] view at source ↗
read the original abstract

We discuss the possibility of using the observables deduced from the angular distribution of the $B\to D^{(\ast)} \ell\bar{\nu}$ decays to test the effects of lepton flavor universality violation (LFUV). We show that the measurement of even a subset of these observables could be very helpful in distinguishing the Lorentz structure of the New Physics contributions to these decays. To do so we use the low energy effective theory in which besides the Standard Model contribution we add all possible Lorentz structures with the couplings (Wilson coefficients) that are determined by matching theory with the measured ratios $R{(D^{(\ast)})}^\mathrm{exp}$. We argue that even in the situation in which the measured $R{(D^{(\ast)})}^\mathrm{exp}$ becomes fully compatible with the Standard Model, one can still have significant New Physics contributions the size of which could be probed by measuring the observables discussed in this paper and comparing them with their Standard Model predictions.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The paper claims that angular observables extracted from the differential distributions of B̄ → D(*) ℓ ν̄ decays can distinguish the Lorentz structure of New Physics contributions to b → c τ ν transitions. Wilson coefficients of all possible operators (vector, scalar, tensor) are fitted to the measured R(D(*)) ratios; the resulting parameter space is then used to predict angular observables, with the central assertion that significant NP effects remain detectable in the angular distributions even in the limit where R(D(*)) becomes fully compatible with the Standard Model.

Significance. If the explicit calculations confirm that flat directions in the multi-operator fit produce observable deviations in angular observables while preserving R(D(*)) ≈ R(D(*))_SM, the work would supply a concrete, falsifiable roadmap for future Belle II and LHCb measurements to resolve the operator structure of any LFUV. The use of the complete low-energy effective Lagrangian is a strength, as is the emphasis on observables that are in principle accessible with current detector capabilities.

major comments (2)
  1. [§3 (fitting procedure) and §4 (angular predictions)] The central claim (abstract and §4) that non-zero Wilson coefficients can reproduce R(D(*)) ≈ SM while generating large angular deviations rests on the existence of allowed directions in the multi-dimensional WC space. With five independent coefficients for the τ mode and only two integrated constraints, the fit necessarily admits flat directions; the manuscript must explicitly map those directions, quantify the size of the resulting angular deviations (e.g., in the forward-backward asymmetry or the D* polarization observables), and demonstrate that they exceed experimental precision. Without this mapping the claim that angular observables remain sensitive when R(D(*)) matches the SM is not yet load-bearing.
  2. [§3] The paper fits exclusively to R(D) and R(D*), which are ratios of integrated rates. It is not shown whether additional theoretical constraints (perturbative unitarity, matching to a UV completion, or bounds from other b → c ℓ ν observables) further restrict the degenerate directions that would otherwise produce large angular effects. If those directions are excluded by such constraints, the phenomenological reach of the angular observables is reduced.
minor comments (2)
  1. [§2] Notation for the Wilson coefficients (C_V, C_S, C_T, …) should be defined once in a table with explicit normalization conventions relative to the SM operator.
  2. [§4] The SM predictions for the angular observables are stated but the numerical values and their theoretical uncertainties (form-factor errors, etc.) should be collected in a dedicated table for direct comparison with the NP scenarios.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful reading and constructive comments on our manuscript. We address the major comments point by point below. Where the referee correctly identifies gaps in the explicit demonstration of flat directions, we will revise the manuscript to incorporate the requested mapping and quantification.

read point-by-point responses

  1. Referee: [§3 (fitting procedure) and §4 (angular predictions)] The central claim (abstract and §4) that non-zero Wilson coefficients can reproduce R(D(*)) ≈ SM while generating large angular deviations rests on the existence of allowed directions in the multi-dimensional WC space. With five independent coefficients for the τ mode and only two integrated constraints, the fit necessarily admits flat directions; the manuscript must explicitly map those directions, quantify the size of the resulting angular deviations (e.g., in the forward-backward asymmetry or the D* polarization observables), and demonstrate that they exceed experimental precision. Without this mapping the claim that angular observables remain sensitive when R(D(*)) matches the SM is not yet load-bearing.

    Authors: We agree that the central claim would be strengthened by an explicit mapping of the flat directions. The manuscript currently illustrates the effect via benchmark points in the WC space that keep R(D(*)) near SM values while shifting angular observables, but does not systematically chart the degenerate directions or compare deviations to projected experimental precision. In the revised version we will add this analysis, identifying the allowed ranges in the five-dimensional WC space (for τ modes) consistent with R(D(*)) within current uncertainties and quantifying the resulting shifts in A_FB, F_L^{D*}, and related observables against Belle II/LHCb sensitivities. revision: yes

  2. Referee: [§3] The paper fits exclusively to R(D) and R(D*), which are ratios of integrated rates. It is not shown whether additional theoretical constraints (perturbative unitarity, matching to a UV completion, or bounds from other b → c ℓ ν observables) further restrict the degenerate directions that would otherwise produce large angular effects. If those directions are excluded by such constraints, the phenomenological reach of the angular observables is reduced.

    Authors: The work is performed in the general low-energy EFT, where the only data-driven constraints used are the measured R(D(*)) ratios; additional restrictions from perturbative unitarity, UV matching, or other b→cℓν observables are model-dependent and therefore omitted. We will add a short discussion noting that specific UV completions may further restrict the flat directions, but the EFT analysis demonstrates the discriminating power of angular observables in the most general case allowed by existing R(D(*)) data. revision: partial

Circularity Check

0 steps flagged

No significant circularity; derivation remains self-contained

full rationale

The paper defines an EFT framework that includes all Lorentz structures, fits the associated Wilson coefficients exclusively to the two integrated ratios R(D(*)), and then computes angular observables as functions of those coefficients. The angular distributions are independent observables not entering the fit, so their expressions are not equivalent to the input ratios by construction. The central argument—that angular observables can still reveal NP even if future R(D(*)) measurements become SM-compatible—rests on the structure of the multi-operator EFT rather than a tautological re-derivation of the fitted values. No self-definitional steps, load-bearing self-citations, or renaming of known results appear in the provided text.

Axiom & Free-Parameter Ledger

1 free parameters · 2 axioms · 0 invented entities

The central claim rests on the validity of the low-energy effective theory containing all Lorentz structures and on the assumption that R(D(*)) data can be used to fix the Wilson coefficients without contamination from other observables or higher-scale effects.

free parameters (1)
  • Wilson coefficients for each Lorentz structure
    Fitted to the measured R(D(*))exp values; these are the adjustable parameters that encode the strength of each new physics contribution.
axioms (2)
  • domain assumption Low-energy effective field theory with all possible Lorentz structures is sufficient to describe the decays
    Invoked when adding NP contributions beyond the SM and matching to data.
  • domain assumption No significant contributions from operators outside the considered set or from other decay modes
    Implicit in the statement that coefficients are determined solely by R(D(*)) matching.

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Forward citations

Cited by 1 Pith paper

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score.

  1. New physics searches via angular distributions of $ \bar{B} \to D^* (\to D \pi) \tau (\to \ell \nu_\tau \bar{\nu}_\ell) \bar{\nu}_\tau$ decays

    hep-ph 2024-11 unverdicted novelty 5.0

    Introduces a reconstructible angular distribution for B to D* tau nu decays via tau to lepton chain to extract new physics parameters, projecting 5-6% sensitivities from simulation and lattice form factors.

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