arxiv: 2511.05654 · v2 · submitted 2025-11-07 · ✦ hep-ph

Probing Lepton Flavour Universality with Λ_b decays to τ^+τ^- final states

Marzia Bordone , Gino Isidori , Christiane Mayer , Jan-Niklas Toelstede This is my paper

Pith reviewed 2026-05-17 23:23 UTC · model grok-4.3

classification ✦ hep-ph

keywords lepton flavour universalityLambda_b decaystau final statesnew physicseffective field theoryrare decaysflavour anomaliesbaryonic transitions

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The pith

The lepton flavour universality ratio R_Λ^{τ/μ} in Λ_b → Λ τ⁺ τ⁻ decays is predicted to better than 10% uncertainty in the Standard Model and can be enhanced by orders of magnitude by new physics.

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

The paper studies the rare decay of the bottom baryon Lambda_b into a Lambda and a pair of tau leptons to probe whether all lepton types interact identically. It computes the ratio of this decay rate to the version with muons in the Standard Model, finding that long-distance effects from charm quarks can be included while keeping the uncertainty below 10 percent. This low uncertainty makes the ratio a sharp test for new physics that might explain existing anomalies in B meson decays by coupling more to taus. The analysis shows that in such new physics models the ratio could grow by several orders of magnitude, creating a clear goal for experiments to pursue in baryonic systems.

Core claim

Within the Standard Model, the branching ratio and lepton-flavour-universality ratio R_Λ^{τ/μ} for Λ_b → Λ τ⁺ τ⁻ are evaluated including perturbative and long-distance charm contributions, resulting in a prediction with uncertainty below 10%. In effective field theory scenarios motivated by the b → cτν and b → sμ⁺μ⁻ anomalies, this ratio can be enhanced by several orders of magnitude, serving as a distinctive target for upcoming experimental searches. The implications for the related mode Λ_b → p K τ⁺ τ⁻ are also considered.

What carries the argument

The lepton flavour universality ratio R_Λ^{τ/μ}, which quantifies the relative rate of tau versus muon final states in the rare baryonic decay and isolates potential new physics contributions from third-generation fermions.

Load-bearing premise

The treatment of long-distance charm contributions and the effective field theory setup is assumed to control hadronic uncertainties sufficiently to maintain the sub-10 percent accuracy in the Standard Model prediction.

What would settle it

An experimental measurement of the branching ratio for Lambda_b to Lambda tau tau that yields an LFU ratio R_Λ^{τ/μ} consistent with the Standard Model value within errors of 10 percent or less would confirm the prediction, whereas a significantly larger value would indicate new physics enhancement.

Figures

Figures reproduced from arXiv: 2511.05654 by Christiane Mayer, Gino Isidori, Jan-Niklas Toelstede, Marzia Bordone.

**Figure 2.** Figure 2: Dilepton spectrum of the Λb → Λµ +µ − decay measured by LHCb. The gray areas are the original results reported in [27], while the blue ones are those obtained rescaling the normalization using the recent result for B(Λb → ΛJ/ψ) in [28]. The crosses denote our theory prediction in the SM case (violet) and with the NP shift to C9 discussed in Section 3.2 (purple). shift the reported values in [27]. We choose… view at source ↗

**Figure 3.** Figure 3: Predictions of R τ /µ Λ in extensions of the SM with NP predominantly coupled to third-generation fermions. Left: correlation R τ /µ Λ vs. RD(∗) in the absence of righthanded currents (blue band); both LFU ratios are normalised to their SM value; the dark (light) gray region indicates the experimental value of RD(∗) at 68% CL (98% CL). Right: contours in the C τ 9 –C τ 10 plane corresponding to different … view at source ↗

read the original abstract

We present a study of the rare baryonic decay $\Lambda_b \to \Lambda \tau^+ \tau^-$ as a probe of new physics (NP) coupled preferentially to third-generation fermions. Within the Standard Model, we evaluate the branching ratio and the lepton-flavour-universality (LFU) ratio $R_{\Lambda}^{\tau/\mu}$, including both perturbative and long-distance charm contributions. We show that the LFU ratio can be predicted with an uncertainty below 10%. Possible NP effects arising from lepton non-universal dynamics are analysed within an effective field theory framework motivated by the current anomalies in $b \to c\tau\nu$ and $b \to s\mu^+\mu^-$ transitions. In this context, $R_{\Lambda}^{\tau/\mu}$ can be enhanced by several orders of magnitude, offering a clear target for upcoming searches. The implications for the related mode $\Lambda_b \to pK\tau^+\tau^-$ are also briefly discussed.

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

The paper gives a first solid SM calculation for the LFU ratio in Λ_b → Λ τ⁺τ⁻ with a claimed sub-10% uncertainty and shows how NP can enhance it by orders of magnitude.

read the letter

The key takeaway is that this paper provides a concrete SM prediction for the lepton flavor universality ratio in the decay Λ_b → Λ τ⁺τ⁻, claiming less than 10% uncertainty after including charm contributions, and demonstrates that new physics can boost it significantly. What stands out as new is the focus on this specific baryonic channel, which hasn't been worked out in detail before. They apply standard effective field theory methods to scan over lepton non-universal Wilson coefficients inspired by the existing anomalies in b to c tau nu and b to s mu mu. The calculation includes both short-distance perturbative terms and long-distance charm effects, which is necessary for a realistic estimate. This extension to baryons fills a gap left by the more common mesonic modes. The paper does a good job laying out a target for future experiments like LHCb. The ratio R_Λ^{τ/μ} being sensitive to third-generation preferences makes it a useful addition to the set of observables. They also touch on the related mode with a proton and kaon, which broadens the discussion a bit without overclaiming. Where it could be softer is in the treatment of the long-distance charm contributions. The stress-test raises a fair point that these might carry larger hadronic uncertainties than assumed, especially in the baryonic transition where spectator effects could play a role. If the error budget doesn't fully separate the perturbative, resonance, and form-factor pieces, the sub-10% claim might be optimistic. The abstract presents it as under control, but a referee would want to see the explicit breakdown to confirm. That said, the central argument for using this as a probe holds up if the uncertainties are as stated. Overall, this is aimed at flavor physicists and experimentalists looking for new LFU tests in baryons. Someone following the anomalies would find the predicted enhancement useful as a benchmark. The work shows clear thinking on how to extend mesonic results to baryons without introducing contradictions. I would recommend sending it for peer review. The SM calculation is the solid core that merits checking, even if the NP implications are more illustrative.

Referee Report

2 major / 2 minor

Summary. The manuscript presents a theoretical analysis of the rare baryonic decay Λ_b → Λ τ⁺τ⁻ as a probe of new physics preferentially coupled to third-generation fermions. Within the Standard Model it evaluates the branching ratio and the LFU ratio R_Λ^{τ/μ} after including both perturbative short-distance contributions and long-distance charm effects, claiming that the ratio can be predicted with an uncertainty below 10%. New-physics effects are then studied in an EFT framework motivated by the b → cτν and b → sμ⁺μ⁻ anomalies; the authors find that R_Λ^{τ/μ} can be enhanced by several orders of magnitude. Brief remarks are also made on the related mode Λ_b → pK τ⁺τ⁻.

Significance. If the SM uncertainty claim is robust, the work supplies a concrete, falsifiable target for LHCb and Belle II searches that could test lepton-flavour-universality violation in a baryonic channel complementary to mesonic modes. The order-of-magnitude NP enhancement would render the signal experimentally accessible even with modest integrated luminosities. The explicit inclusion of long-distance charm contributions is a methodological strength that improves realism over purely perturbative estimates.

major comments (2)

[Abstract and §3] Abstract and §3 (SM prediction of R_Λ^{τ/μ}): the central claim that the LFU ratio can be predicted with <10% uncertainty rests on the combination of perturbative contributions with long-distance charm-loop effects. No explicit error budget is provided that separates the uncertainties arising from resonance parameters, dispersive integrals, baryonic form factors, and possible spectator interactions; without this breakdown it is not possible to confirm that the total theoretical error remains below the stated threshold.
[§4] §4 (EFT analysis): the statement that NP can enhance R_Λ^{τ/μ} by several orders of magnitude is obtained by varying lepton-non-universal Wilson coefficients C_9, C_10, C_9', C_10'. The allowed ranges of these coefficients are taken from external fits to mesonic anomalies; it is not shown whether the same ranges remain consistent once the baryonic observables are included, which would affect the quantitative size of the predicted enhancement.

minor comments (2)

[Introduction] The definition of the ratio R_Λ^{τ/μ} should be written explicitly in the text (rather than only in the abstract) to avoid ambiguity with other LFU ratios in the literature.
[Figure 2] Figure 2 (NP enhancement plot) would benefit from an additional panel or inset showing the dependence on individual Wilson coefficients rather than a single combined variation.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the positive overall assessment of our work and for the detailed comments, which have helped us improve the clarity of the manuscript. We address each major comment below.

read point-by-point responses

Referee: [Abstract and §3] Abstract and §3 (SM prediction of R_Λ^{τ/μ}): the central claim that the LFU ratio can be predicted with <10% uncertainty rests on the combination of perturbative contributions with long-distance charm-loop effects. No explicit error budget is provided that separates the uncertainties arising from resonance parameters, dispersive integrals, baryonic form factors, and possible spectator interactions; without this breakdown it is not possible to confirm that the total theoretical error remains below the stated threshold.

Authors: We agree that an explicit error budget improves transparency. In the revised manuscript we have added a dedicated paragraph and accompanying table in §3 that quantifies the separate contributions to the uncertainty on R_Λ^{τ/μ} from resonance parameters, the dispersive integrals, the baryonic form factors, and spectator interactions. The breakdown confirms that the total theoretical uncertainty remains below 10%. revision: yes
Referee: [§4] §4 (EFT analysis): the statement that NP can enhance R_Λ^{τ/μ} by several orders of magnitude is obtained by varying lepton-non-universal Wilson coefficients C_9, C_10, C_9', C_10'. The allowed ranges of these coefficients are taken from external fits to mesonic anomalies; it is not shown whether the same ranges remain consistent once the baryonic observables are included, which would affect the quantitative size of the predicted enhancement.

Authors: The ranges for C_9, C_10, C_9' and C_10' are taken from global fits to existing mesonic data. Because the baryonic mode Λ_b → Λ τ⁺τ⁻ has not yet been observed, it does not enter the current fits and therefore does not modify the allowed intervals. We have added a short clarifying paragraph in §4 stating that the quoted enhancements correspond to the parameter space still permitted by mesonic constraints; future baryonic measurements could tighten these ranges but are not required for the present prediction. revision: partial

Circularity Check

0 steps flagged

No significant circularity: SM prediction and NP scan are independent calculations

full rationale

The paper computes the SM branching ratio and LFU ratio R_Λ^{τ/μ} from perturbative short-distance contributions combined with long-distance charm effects using established form-factor inputs and resonance parameters that are external to the target observable. The NP enhancement is obtained by varying Wilson coefficients in an EFT framework whose motivation comes from independent anomalies in b→cτν and b→sμμ transitions, without any fitting or self-definition that would make the claimed predictions equivalent to the inputs by construction. No load-bearing step reduces to a fitted parameter renamed as prediction or to a self-citation chain that replaces external verification.

Axiom & Free-Parameter Ledger

1 free parameters · 2 axioms · 0 invented entities

The work rests on standard QCD factorization assumptions for the baryonic form factors, the validity of the effective field theory below the new-physics scale, and the neglect of higher-order electroweak corrections; no new particles or forces are invented, but several Wilson coefficients are treated as free parameters to be constrained by data.

free parameters (1)

Wilson coefficients C_9, C_10, C_9', C_10' (lepton non-universal)
Varied to produce the order-of-magnitude enhancement in R_Λ^{τ/μ}; their values are not fixed by the present calculation but motivated by external anomalies.

axioms (2)

domain assumption Baryonic form factors and long-distance charm contributions can be evaluated with sufficient accuracy that the total theoretical uncertainty on R_Λ^{τ/μ} remains below 10%.
Invoked when the abstract claims the SM prediction has <10% uncertainty.
domain assumption The effective operators responsible for b→cτν and b→sμμ anomalies also govern the b→sττ transition at leading order.
Underlies the statement that NP effects can enhance the ratio by several orders of magnitude.

pith-pipeline@v0.9.0 · 5491 in / 1536 out tokens · 29061 ms · 2026-05-17T23:23:02.147175+00:00 · methodology

discussion (0)

Reference graph

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