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arxiv: 1907.01500 · v1 · pith:N7PPV2AYnew · submitted 2019-07-02 · ✦ hep-ex

Lepton flavour universality in charged-current B decays

Suzanne Klaver (on behalf of the LHCb collaboration) This is my paper

Pith reviewed 2026-05-25 10:15 UTC · model grok-4.3

classification ✦ hep-ex
keywords lepton flavour universalityB decaysR(D)R(D*)LHCbBelleStandard Model testssemileptonic decays
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The pith

Combined measurements of R(D) and R(D*) deviate by 3.1 sigma from Standard Model predictions.

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

The paper reports LHCb results on the ratios R(D*) and R(J/ψ) in charged-current B decays, together with Belle's recent semileptonic-tag measurements of R(D) and R(D*). These ratios compare branching fractions for tau versus light-lepton final states and thereby test whether the weak interaction treats all lepton flavours equally. The latest HFLAV average of all available data yields a 3.1 sigma discrepancy with Standard Model expectations. A sympathetic reader would care because sustained deviations could signal new particles or forces that couple preferentially to the third lepton generation. The work therefore supplies updated experimental inputs for testing a core symmetry assumption of the Standard Model.

Core claim

LHCb measures R(D*) and R(J/ψ) while Belle measures R(D) and R(D*); when these and earlier results are averaged by HFLAV the combined values for R(D) and R(D*) lie 3.1 sigma above the Standard Model predictions.

What carries the argument

The ratios R(D), R(D*) and R(J/ψ) that quantify lepton-flavour-universality violation by comparing tau and light-lepton branching fractions in b to c transitions.

Load-bearing premise

The Standard Model theoretical predictions for R(D) and R(D*) are accurate and the experimental results from LHCb and Belle can be combined without unaccounted systematic differences.

What would settle it

A new measurement or updated average that brings the combined R(D) and R(D*) values within 2 sigma of the Standard Model prediction, or a revised theory calculation that raises the predicted central values to match the experimental average.

Figures

Figures reproduced from arXiv: 1907.01500 by Suzanne Klaver (on behalf of the LHCb collaboration).

Figure 1
Figure 1. Figure 1: FIG. 1: Distributions of m [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2: Fit projections of the three-dimensional fit of the 3 [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3: Fit projections of the [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4: Distributions of the [PITH_FULL_IMAGE:figures/full_fig_p004_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: FIG. 5: HFLAV average of all measurements of [PITH_FULL_IMAGE:figures/full_fig_p005_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: FIG. 6: Fit projections of the [PITH_FULL_IMAGE:figures/full_fig_p006_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: FIG. 7: Fit projections of the [PITH_FULL_IMAGE:figures/full_fig_p006_7.png] view at source ↗
read the original abstract

Tests of lepton flavour universality in charged-current $B$ decays offer an excellent opportunity to test the Standard Model, and show hints of new physics in analyses performed by the LHCb, Belle and BaBar experiments. These proceedings present the results from the LHCb collaboration on measurements of $\mathcal{R}(D^*)$ and $\mathcal{R}(J/\psi)$. It also presents the latest semileptonic tag measurement of $\mathcal{R}(D)$ and $\mathcal{R}(D^*)$ by the Belle collaboration. The latest HFLAV average shows a discrepancy of 3.1$\sigma$ between the Standard Model predictions and combined measurements of $\mathcal{R}(D)$ and $\mathcal{R}(D^*)$.

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

1 major / 0 minor

Summary. This proceedings paper reports LHCb measurements of R(D*) and R(J/ψ) along with Belle semileptonic-tag results for R(D) and R(D*) in charged-current B decays. It states that the latest HFLAV average of R(D) and R(D*) measurements exhibits a 3.1σ tension with Standard Model predictions.

Significance. Confirmation of a genuine deviation from lepton flavour universality in b→c transitions would constitute significant evidence for physics beyond the Standard Model, with direct implications for models involving leptoquarks or other mediators that preferentially couple to third-generation fermions.

major comments (1)
  1. [Abstract] The central claim of a 3.1σ discrepancy is taken directly from the external HFLAV average without any discussion or citation of the covariance matrix, treatment of shared form-factor inputs, or possible correlations in efficiency and background modelling between the LHCb and Belle datasets; this assumption is load-bearing for the quoted significance (abstract).

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their review of our proceedings contribution. We address the single major comment below.

read point-by-point responses

  1. Referee: [Abstract] The central claim of a 3.1σ discrepancy is taken directly from the external HFLAV average without any discussion or citation of the covariance matrix, treatment of shared form-factor inputs, or possible correlations in efficiency and background modelling between the LHCb and Belle datasets; this assumption is load-bearing for the quoted significance (abstract).

    Authors: This is a short proceedings paper that reports the individual measurements from LHCb and Belle and notes the published HFLAV world average. The 3.1σ significance and the details of the combination, including the covariance matrix, treatment of form factors, and inter-experiment correlations, are provided in the HFLAV reference. We do not perform the average ourselves. We will ensure the appropriate HFLAV citation is included in the revised version if it is not already present. The correlations between the LHCb and Belle datasets are handled within the HFLAV combination procedure. revision: partial

Circularity Check

0 steps flagged

No circularity: direct experimental report citing external average

full rationale

This proceedings paper reports LHCb measurements of R(D*) and R(J/ψ) and cites Belle semileptonic-tag results plus the external HFLAV average for the 3.1σ discrepancy with SM predictions. No equations, derivations, or predictions are presented that reduce by construction to inputs defined inside the paper. The discrepancy significance is taken directly from the cited HFLAV combination (external to this work) with no self-citation load-bearing or fitted-parameter renaming. The paper is self-contained as an experimental summary against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The central claim rests on the accuracy of existing SM calculations for the ratios and on the validity of the experimental reconstruction and background subtraction techniques standard in the field; no new free parameters or invented entities are introduced in the abstract.

axioms (2)
  • domain assumption Standard Model predictions for R(D) and R(D*) are correctly calculated from first principles or lattice QCD.
    The 3.1σ discrepancy is defined relative to these predictions.
  • domain assumption Experimental efficiencies and background models in LHCb and Belle analyses are unbiased.
    Required to interpret the measured ratios as true branching-fraction ratios.

pith-pipeline@v0.9.0 · 5643 in / 1215 out tokens · 25170 ms · 2026-05-25T10:15:55.736382+00:00 · methodology

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Reference graph

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16 extracted references · 16 canonical work pages · 13 internal anchors

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