arxiv: 2604.26879 · v1 · submitted 2026-04-29 · ✦ hep-ex

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Flavour changing charged current decays at LHCb

Davide Fazzini

Authors on Pith no claims yet

Pith reviewed 2026-05-07 10:38 UTC · model grok-4.3

classification ✦ hep-ex

keywords LHCbsemileptonic decayslepton universalityR(D**)form factorsbranching fractionsb-hadron decayscharged currents

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The pith

LHCb has performed the first measurement of the ratio R(D**) in B meson decays to tau leptons and extracted related form factor and branching fraction results.

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

This paper reports three measurements from the LHCb experiment on semileptonic decays of b-hadrons that proceed through charged weak currents. The first result is the initial experimental value of the ratio R(D**), which compares the branching fraction for decays involving tau leptons to those with lighter leptons in transitions to excited charm states. The second is a determination of the branching fraction for the decay of the Lambda baryon into a proton, a muon, and an antineutrino. The third extracts parameters that describe the hadronic form factors in the decay of neutral B mesons to a D-star meson, a muon, and a neutrino. These observables test the standard model's assumption that the weak force couples with equal strength to all lepton flavors and supply data that can reveal effects from new physics.

Core claim

The LHCb collaboration has made the first measurement of the ratio of branching fractions R(D**) using B- to D**0 tau- anti-nu_tau decays, determined the branching fraction for Lambda to p mu- anti-nu_mu, and extracted form-factor parameters from B0 to D*- mu+ nu_mu decays. These results test lepton universality in charged-current interactions of b-hadrons.

What carries the argument

The ratio R(D**), defined as the branching fraction for B decays to excited D states with tau leptons divided by the corresponding rate with light leptons, serves as the primary probe for possible violations of lepton flavor universality.

If this is right

The measured R(D**) can be compared to standard model predictions to check for deviations indicating new physics.
The Lambda baryon branching fraction provides a reference value for studies of semileptonic decays in the baryon sector.
The extracted form-factor parameters allow more precise theoretical predictions for other b to c transition processes.
The results together constrain models of new physics that couple differently to tau leptons than to electrons or muons.

Where Pith is reading between the lines

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

Combining the new R(D**) result with earlier measurements of R(D) and R(D*) could test whether any observed deviations follow a consistent pattern across different charm final states.
The form factor parameters may enable sharper comparisons with lattice QCD calculations to isolate potential new physics contributions.
The baryonic decay measurement suggests that Lambda-based channels could serve as a complementary probe with different systematic uncertainties from mesonic decays.

Load-bearing premise

The analyses assume that detector efficiencies, background shapes, and simulation models accurately represent the data without introducing significant biases.

What would settle it

An independent measurement of R(D**) on a separate dataset or by another experiment that differs from the reported central value by more than the combined statistical and systematic uncertainties.

read the original abstract

The Standard Model (SM) predicts the universality of lepton couplings with the electroweak gauge bosons. Semileptonic decays of $b$-hadrons provide a powerful framework for testing the SM and probing possible New Physics effects. In particular, the processes mediated by charged-current interactions benefit from a relatively large branching fractions and theoretically well-controlled hadronic matrix elements. This contribution presents three recent results from the LHCb experiment: the first measurement of the ratio of branching fractions $\mathcal{R}(D^{**})$ using $B^{-} \to D^{**0} \tau^{-} \bar{\nu}_{\tau}$ decays, the determination of the branching fraction for $\Lambda \to p \mu^{-} \bar{\nu}_{\mu}$ and the extraction of form-factor parameters from $B^0 \to D^{*-} \mu^{+} \nu_{\mu}$ decays.

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.

Desk Editor's Note private letter to a colleague

Short LHCb summary with one new R(D**) ratio but thin on the usual validation checks.

read the letter

This contribution is a compact report of three LHCb results on charged-current semileptonic b-hadron decays. The genuinely new item is the first measurement of R(D**), the ratio involving B- to D**0 tau nu over the muonic mode. The other two pieces, the Lambda to p mu nu branching fraction and the B0 to D* mu nu form-factor parameters, are standard extractions that build on earlier work but still add numbers to the literature. The context section does a clear job of placing these in the ongoing tests of lepton universality, where small deviations have drawn attention. That framing is useful for readers who follow the field. The main limitation is the format itself. As a short contribution there is no space for the data-MC comparison plots, sideband checks, or efficiency validation that normally let outsiders judge how well the simulation captures efficiencies, backgrounds, and tau reconstruction. The stress-test note about assumed fidelity is therefore on target for what is shown here; any mismatch in those areas would shift the central values. This is the sort of material flavor physicists track for updates on existing anomalies, but it is not yet a self-contained analysis paper. A reader looking for the full systematic breakdown would need the longer write-up that usually follows these conference summaries. I would bring it to a reading group if the group is already following LHCb semileptonic results, mainly to note the new R(D**) number and flag that the details are still pending. It is worth sending to peer review once the full paper with the validation material is ready, because the topic is active and the result is timely.

Referee Report

2 major / 0 minor

Summary. The manuscript reports three recent LHCb measurements on semileptonic b-hadron decays mediated by charged currents: the first determination of the ratio of branching fractions R(D**) in B− → D**0 τ− ν¯τ decays, the branching fraction for Λ → p μ− ν¯μ, and the extraction of form-factor parameters from B0 → D*− μ+ νμ decays. These results are presented as tests of lepton universality and inputs for hadronic matrix elements.

Significance. If substantiated, the R(D**) result would be the first experimental constraint on this ratio and could inform the interpretation of R(D(*)) anomalies; the Λ branching fraction adds to the limited data on baryonic semileptonic decays; and the form-factor parameters improve the precision of |Vcb| extractions. The work therefore addresses timely questions in heavy-flavour physics with direct experimental input.

major comments (2)

The contribution provides no quantitative information on data-MC agreement for tau identification, missing-mass reconstruction, or background shapes in the R(D**) analysis, nor on tag-and-probe efficiency ratios or sideband fit quality for the other channels. Without these cross-checks the central values and uncertainties cannot be verified as unbiased.
The manuscript states that the analyses rely on simulation for detector efficiencies, kinematic acceptance, and background modelling, yet supplies no tables or figures quantifying the associated systematic uncertainties or their impact on the quoted results.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful review and constructive comments on our manuscript, which summarises three recent LHCb results on charged-current semileptonic decays. We address each major comment below and indicate the revisions we will make.

read point-by-point responses

Referee: The contribution provides no quantitative information on data-MC agreement for tau identification, missing-mass reconstruction, or background shapes in the R(D**) analysis, nor on tag-and-probe efficiency ratios or sideband fit quality for the other channels. Without these cross-checks the central values and uncertainties cannot be verified as unbiased.

Authors: This manuscript is a concise overview of three separate analyses, each published (or submitted) in dedicated papers that contain the requested quantitative studies. The R(D**) result is detailed in a full LHCb paper that includes data-MC comparisons for tau identification, missing-mass distributions, and background modelling; the Lambda_b branching fraction paper shows tag-and-probe efficiency ratios and sideband fits; and the B0 to D* form-factor extraction includes the corresponding validation plots. We will revise the text to add explicit references to these publications and a short paragraph directing readers to the relevant figures and tables therein. Length limits of the contribution preclude reproducing the full set of validation plots. revision: partial
Referee: The manuscript states that the analyses rely on simulation for detector efficiencies, kinematic acceptance, and background modelling, yet supplies no tables or figures quantifying the associated systematic uncertainties or their impact on the quoted results.

Authors: The dominant systematic uncertainties and their breakdown are presented in the individual analysis papers. To improve clarity in this summary, we will add a compact table listing the leading systematic sources for each of the three measurements together with their relative impact on the central values. This table will be derived directly from the published results. revision: yes

Circularity Check

0 steps flagged

No circularity: direct experimental measurements with no derivation chain

full rationale

The paper reports three LHCb measurements: R(D**) from B- → D**0 τ- ντ decays, Br(Λ → p μ- νμ), and form-factor parameters from B0 → D*- μ+ νμ. These are extracted via standard data analysis techniques (efficiencies, background subtraction, fits to data). No theoretical derivation, ansatz, or prediction is presented that reduces by construction to its own inputs or to a self-citation. The contribution contains no equations defining quantities in terms of themselves, no fitted parameters renamed as predictions, and no load-bearing uniqueness theorems. All results are falsifiable against independent data samples or other experiments, satisfying the criteria for a non-circular experimental report.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Abstract-only review; no explicit free parameters, axioms, or invented entities are described in the provided text.

pith-pipeline@v0.9.0 · 5436 in / 1035 out tokens · 40285 ms · 2026-05-07T10:38:20.364967+00:00 · methodology

discussion (0)

Reference graph

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