Recognition: unknown
Role of heavy neutral lepton in lepton number violating B meson decays
Pith reviewed 2026-05-08 03:13 UTC · model grok-4.3
The pith
Heavy neutral leptons mediate lepton-number violating B and B_c decays with branching ratios up to 10^{-8}.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
Using constraints from B to lepton N decays, the branching ratios for lepton-number violating B and B_c decays mediated by on-shell heavy neutral leptons reach up to O(10^{-8}) for |U_mu N|^2 = 10^{-6} and M_N = 2-3 GeV, with B_c^- to pi^+ mu^- mu^- having the largest enhancement and B_c^- to J/psi pi^+ mu^- mu^- being strongly suppressed.
What carries the argument
On-shell heavy neutral leptons (HNLs) acting as mediators in Delta L=2 B meson decay channels, with production and decay rates set by their mass and mixing parameters with active neutrinos.
If this is right
- The predicted branching ratios lie in ranges that future B-physics experiments can probe.
- B_c decay modes offer greater sensitivity to these HNL effects than ordinary B modes.
- The strong channel dependence allows experiments to target specific final states for enhanced signals.
- Absence of signals in these channels would tighten the allowed regions for HNL mass and mixing.
Where Pith is reading between the lines
- Searches at LHCb or Belle II for same-sign muon pairs in B_c decays could directly test these rates.
- Confirmation would strengthen the case for HNLs as a source of lepton number violation.
- The same on-shell mediation approach could be applied to other meson systems or lepton flavors.
Load-bearing premise
Heavy neutral leptons exist in the GeV mass range, are produced on-shell in B decays, and mediate the lepton number violating processes with mixing values taken from regions allowed by B to mu N and B to tau N data.
What would settle it
A precise measurement of the branching ratio for B_c^- to pi^+ mu^- mu^- that lies well outside the predicted O(10^{-13}) to O(10^{-8}) range for the benchmark |U_mu N|^2 = 10^{-6} and M_N = 2-3 GeV would falsify the central prediction.
Figures
read the original abstract
We study the phenomenology of heavy neutral leptons (HNLs) in $B$-meson decays as probes of physics beyond the Standard Model. Focusing on the leptonic channels $B \to \mu N$ and $B \to \tau N$, we constrain the allowed regions in the $M_N$--$|U_{\ell N}|^2$ plane using current experimental data. Using these constraints, we investigate lepton-number violating ($\Delta L=2$) processes mediated by on-shell HNLs, including $B_{(c)}^- \to \pi^+ \mu^- \mu^-$ and $B_c^- \to J/\psi\, \pi^+ \mu^- \mu^-$. For benchmark values $|U_{\mu N}|^2 = 10^{-6}$ and $M_N = 2$-- $3\,\mathrm{GeV}$, the predicted branching ratios lie in the range $\mathcal{O}(10^{-13})$--$\mathcal{O}(10^{-8})$. Among the channels, $B_c^- \to \pi^+ \mu^- \mu^-$ shows the largest enhancement, while $B_c^- \to J/\psi\, \pi^+ \mu^- \mu^-$ is strongly suppressed. These results indicate a clear channel dependence, with $B_c$ modes providing enhanced sensitivity to HNL effects and offering promising avenues for future searches of lepton number violation.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript examines heavy neutral leptons (HNLs) as mediators of lepton-number violating (ΔL=2) B and B_c meson decays. It first derives constraints on the mixing parameters |U_ℓN|^2 (ℓ=μ,τ) and M_N from existing data on the two-body decays B→ℓN. These constraints are then used to compute branching ratios for the three-body LNV channels B^-→π^+μ^-μ^-, B_c^-→π^+μ^-μ^- and B_c^-→J/ψ π^+ μ^- μ^- assuming on-shell HNL exchange. For the benchmark point |U_μN|^2=10^{-6} and M_N=2–3 GeV the predicted branching ratios fall in the range O(10^{-13})–O(10^{-8}), with the largest values in the B_c→πμμ channel and strong suppression in the J/ψ mode.
Significance. If the numerical results hold, the work supplies concrete, channel-dependent benchmarks that can directly inform experimental searches for LNV at LHCb and Belle II. The observation that B_c modes are more sensitive than ordinary B modes is a useful phenomenological insight. The calculations rely on standard narrow-width and form-factor techniques already employed in the HNL literature, so the main value lies in the updated constraints and the explicit comparison across channels rather than in novel methodology.
minor comments (3)
- The abstract and main text quote an overall branching-ratio range O(10^{-13})–O(10^{-8}) but do not tabulate the individual values for each channel at the benchmark points; a compact table would improve readability and allow direct comparison with experimental sensitivities.
- The text states that constraints are taken from 'current experimental data' on B→μN and B→τN; explicit citation of the specific limits (e.g., LHCb or Belle upper bounds) and a brief description of how they are translated into the |U_ℓN|^2–M_N plane would strengthen the presentation.
- The suppression of the B_c^-→J/ψ π^+ μ^- μ^- channel is attributed to kinematics and hadronic matrix elements; a short quantitative remark on the relative size of the form-factor suppression versus phase-space effects would clarify the origin of the channel dependence.
Simulated Author's Rebuttal
We thank the referee for their careful reading of the manuscript and for the positive assessment, including the recommendation for minor revision. The referee summary accurately describes the scope, methods, and main findings of our work on HNL-mediated lepton-number violating B and B_c decays. We appreciate the recognition that the channel-dependent branching ratio benchmarks and the enhanced sensitivity of B_c modes constitute useful phenomenological input for LHCb and Belle II searches.
Circularity Check
No significant circularity identified
full rationale
The derivation constrains |U_ℓN|^2 from experimental limits on B → ℓN two-body decays, then evaluates LNV branching ratios at benchmark points inside the allowed region using standard narrow-width approximation, phase-space integration, and conventional hadronic form factors. These steps are independent calculations; the BRs scale with the chosen |U|^2 by the model's Feynman rules but are not fitted to or defined by the LNV observables themselves. No self-citation load-bearing step, ansatz smuggling, or renaming of known results occurs. The paper is self-contained against external data benchmarks.
Axiom & Free-Parameter Ledger
free parameters (2)
- M_N =
2-3 GeV
- |U_μN|^2 =
10^{-6}
axioms (1)
- domain assumption Heavy neutral leptons can be produced on-shell in B-meson decays and mediate lepton-number violating (ΔL=2) processes.
invented entities (1)
-
Heavy neutral lepton (HNL)
no independent evidence
Reference graph
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discussion (0)
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