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Sensitivity of Lepton Number Violating Meson Decays in Different Experiments

T0 review · 3 major / 5 minor · reviewed 2026-08-14 · deepseek-v4-flash

Pith's one-line read Including the parent meson's motion in lepton-number-violating meson-decay searches changes the projected reach by one to two orders of magnitude, with SHiP probing $|V_{eN}|^2\sim10^{-9}$ and MATHUSLA $10^{-7}$.

desk verdict Right that parent-meson boost matters for LNV meson-decay sensitivities, but the projected reaches are idealized and the decay-probability formula omits the production-to-detector baseline for SHiP and MATHUSLA. read the letter →

arxiv 1908.09562 v2 pith:7CNLU3DH submitted 2019-08-26 hep-ph hep-ex

classification hep-phhep-ex PACS 14.60.St13.20.Fc14.60.Pq
keywords leptonnumberviolationheavyMajorananeutrinoright-handedactive-sterilemixinglong-livedparticlesearchesSHiPMATHUSLAmesondecays
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

This paper argues that projected searches for a heavy Majorana neutrino in lepton-number-violating meson decays—decays that produce two same-sign charged leptons and change lepton number by two—must account for the parent meson's motion and the finite detector size, and that the common meson-at-rest approximation misstates the reach by one to two orders of magnitude. The authors model decays such as $D_s^-\to e^-e^-\pi^+$ and $K^-\to e^-e^-\pi^+$ mediated by an on-shell right-handed neutrino $N$ of mass $0.14$-$6$ GeV, and compute the probability that $N$ decays inside SHiP, MATHUSLA, NA62, LHCb, Belle II, and FCC-ee. With the velocity effect included, the future beam-dump experiment SHiP can probe electron-sterile mixing down to $|V_{eN}|^2\sim 10^{-9}$, while MATHUSLA gives the best reach around $|V_{eN}|^2\sim 10^{-7}$ for $N$ masses between 2 and 5 GeV. The point of the calculation is that the same decay chain yields looser bounds when the parent meson is moving, so experiment projections and existing exclusion plots should be read with the boost, not the rest frame, in mind.

What carries the argument

The load-bearing object is the detector-decay probability $P_N=1-\exp(-L_D/L_N)$, with $L_N=p_N/(M_N\Gamma_N)$, together with the boost transformation of the neutrino momentum: in the parent-meson rest frame $p_N^*$ is fixed, but for a meson of momentum $p_{M_1}$ the neutrino energy ranges over $E_N\in[\gamma E_N^*\mp p_N^*\sqrt{\gamma^2-1}]$ with a flat distribution. The paper feeds this $E_N$ range into its master event-counting formula, replacing the rest-frame $P_N$ by an integral over $P_N'=1-\exp(-M_N\Gamma_N L_D/\sqrt{E_N^2-M_N^2})$. This single change—boosting the intermediate neutrino before asking whether it decays in a detector of finite length—is what shifts the projected mixing reach by one to two orders of magnitude. The total width $\Gamma_N=\sum_{\ell}a_\ell(M_N)|V_{\ell N}|^2$ is computed at tree level over pseudoscalar, vector, and three-body channels so the narrow-width factorization stays valid.

What would settle it

A concrete check is to take the measured production-momentum spectrum of $D_s$ mesons at a 400 GeV proton beam dump and recompute the SHiP reach with a full three-dimensional detector geometry and a realistic background count. If the exact boost-averaged decay probability differs from the single-momentum $p_{D_s}=58$ GeV result by less than an order of magnitude, or if even one background event appears in the $D_s\to ee\pi$ signal region, the projected $|V_{eN}|^2\sim 10^{-9}$ reach will not survive.

Watch

Extended reading notes

Core claim

The central claim is that a right-handed Majorana neutrino $N$ in the mass window $0.14\ \text{GeV}\le M_N\le 6\ \text{GeV}$ can be resonantly produced in the three-body LNV decays $M_1^-\to\ell_1^-\ell_2^-\pi^+$ for $M_1=B,B_c,D,D_s,K$, and that the sensitivity of current and proposed experiments to the active-sterile mixings $|V_{\ell N}|^2$ and $|V_{\ell_1 N}V_{\ell_2 N}|$ depends strongly on treating these mesons as moving. The decay rate factorizes under the narrow-width approximation into $\Gamma(M_1\to\ell_1 N)\times \mathrm{Br}(N\to\ell_2\pi^+)$, and the observable signal requires $N$ to decay inside the detector. Because the decay length $L_N=p_N/(M_N\Gamma_N)$ grows with the neutrino momentum, a boosted parent meson produces a harder $N$ spectrum and a smaller decay probability $P_N=1-e^{-L_D/L_N}$; integrating the flat energy distribution of $N$ in the lab frame loosens the bounds by one to two orders of magnitude, most sharply for $D$, $D_s$ at SHiP and $K$ at NA62. With that correction, SHiP's $D_s\to ee\pi$ channel reaches $|V_{eN}|^2\sim 10^{-9}$, MATHUSLA's $B\to ee\pi$ reaches $|V_{eN}|^2\sim 10^{-7}$ for $2\ \text{GeV}<M_N<5\ \text{GeV}$, and LHCb's $B_c$ channels give the tightest tau-mixing constraints in the 5-6 GeV range.

Load-bearing premise

The projected numbers assume zero background events, perfect detection and reconstruction efficiency, a one-dimensional detector length with no angular acceptance, and equal couplings of $N$ to electron, muon, and tau; real detectors that fail any of these will have weaker sensitivities.

Editorial extensions

If this is right

  • Rest-frame projections are unreliable for the forward, boosted experiments: NA62, LHCb, and SHiP each see looser mixing bounds when parent-meson velocity is included, by roughly one order for $B$ and $D_s$ and two for $K$.
  • SHiP's $D_s^-\to e^-e^-\pi^+$ and $D_s^-\to\mu^-\mu^-\pi^+$ channels can probe $|V_{eN}|^2,|V_{\mu N}|^2\sim 10^{-9}$ for $0.14\ \text{GeV}<M_N<1.9\ \text{GeV}$.
  • For $2\ \text{GeV}<M_N<5\ \text{GeV}$, MATHUSLA's $B\to\ell\ell\pi$ modes give the strongest reach, $|V_{eN}|^2,|V_{\mu N}|^2\sim 10^{-7}$, while LHCb's $B_c\to\ell\ell\pi$ modes dominate above 5 GeV.
  • Tau-flavor mixings $|V_{\tau N}|^2$ and the products $|V_{eN}V_{\tau N}|$, $|V_{\mu N}V_{\tau N}|$ can be probed at the $10^{-7}$ level by $B,B_c\to\tau\tau\pi,\ e\tau\pi,\ \mu\tau\pi$ modes at MATHUSLA and LHCb, in mass ranges largely unconstrained by other searches.
  • For $\ell_1\ne\ell_2$, the same-sign final state is both lepton-number and lepton-flavor violating, and the long $N$ lifetime allows the two orderings $M_1\to\ell_1\ell_2\pi$ and $M_1\to\ell_2\ell_1\pi$ to be distinguished experimentally.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • The same boost-averaging correction should apply to any long-lived particle produced in meson decays, not just Majorana neutrinos: a fixed boost or rest-frame approximation will tend to overestimate the visible-decay probability whenever the decay length is comparable to the detector size.
  • The paper's fixed parent momenta (58 GeV for SHiP, 75 GeV for NA62, 100 GeV for LHCb) are proxies; folding in actual production-momentum spectra could widen or narrow the spread, so the rest-versus-in-flight comparison should be repeated per momentum bin before quoting a single combined reach.
  • If the heavy neutrino is a quasi-Dirac pair rather than a pure Majorana state, the $\Delta L=2$ amplitude is suppressed by the mass splitting, so these searches are really measuring the Majorana component; the quoted limits are a maximal-Majorana benchmark rather than a general sterile-neutrino bound.
  • A testable extension: apply the same formalism to heavy neutrinos produced from $W/Z$ decays at MATHUSLA and FCC-ee, where the parent momentum distribution differs, to see whether the velocity correction reshapes those projected reaches as much as it does for meson decays.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

3 major / 5 minor

Summary. This paper studies lepton-number-violating three-body meson decays M1− → ℓ1−ℓ2−π+ mediated by an on-shell right-handed Majorana neutrino N in the mass range 0.14–6 GeV. It computes event rates for NA62, LHCb, Belle II, SHiP, MATHUSLA, and FCC-ee, including the effect of the parent meson boost and a finite detector length, and derives projected sensitivities on |VeN|2, |VμN|2, |VτN|2, and the products |VeNVμN|, |VeNVτN|, |VμNVτN|. The central quantitative claim is that including the parent meson velocity changes the inferred mixing reach by one to two orders of magnitude, with SHiP probing |VeN|2 ∼ 10−9 at low mass and MATHUSLA/LHCb reaching ∼10−7 near 5 GeV.

Significance. The paper is a useful and clearly structured phenomenological survey of LNV meson-decay probes, with explicit partial-width formulas in the appendix and a broad comparison against existing and projected constraints. The observation that parent-meson boost can significantly modify the effective decay length is physically sensible and relevant to how simplified projections are made. The authors are also transparent about several idealizations: the zero-background 3.09-event benchmark, 100% detection and reconstruction efficiency, and the democratic-mixing normalization in the total width are all stated explicitly. If the quantitative results survive a more realistic detector treatment, the paper would provide valuable guidance for the heavy-neutrino search programs at the facilities considered. However, the current sensitivity curves are conditional on a simplified one-dimensional, no-baseline detector model that is not adequate for all of the experiments that drive the headline results.

major comments (3)
  1. [§4, Eqs. (6)–(10); §6] The total decay width ΓN is evaluated with the ad hoc democratic normalization |VeN|2 = |VμN|2 = |VτN|2. Since ΓN appears both in the lifetime and in the branching ratio Γ(N→ℓ2π)/ΓN, the derived limit on a single mixing element such as |VeN|2 from Ds→eeπ is not a bound on that element alone, but a bound in a specific three-flavor-equal scenario. The paper states this assumption explicitly, but it should be justified or its impact quantified, because hierarchical mixings are typical in realistic seesaw realizations and could shift the quoted reaches substantially.
  2. [§6; §8 caveat; Abstract] The projected limits assume zero background and 100% detection and reconstruction efficiency, with the average upper limit set to 3.09 events at 95% C.L. The caveat paragraph in §8 correctly acknowledges the idealized detector, but the abstract and conclusions quote the resulting numbers as the experimental 'sensitivity reach' without this qualification. The headline statements should be explicitly conditional on these assumptions, and for experiments such as NA62 and LHCb the zero-background assumption should at least be motivated by a background estimate. This is a presentation issue as much as a physics issue, but it directly affects how the numbers in the abstract are read.
minor comments (5)
  1. [§5] The text contains several typographical slips, such as 'compare' for 'compared' in the discussion of meson decay in flight; these should be corrected in a revised version.
  2. [Fig. 5] The vertical axis of Fig. 5 is labeled with the symbol ϰ, while the text defines the variable as x = L_D/L_N; the figure should use the same symbol as the text or explicitly define ϰ.
  3. [Fig. 3] The sentence 'Even with mixing angle equal to 1' should read 'mixing squared equal to 1', since the plotted quantity is |VℓN|2.
  4. [§7.1 and §7.5] The Bc meson yield at LHCb and some Belle II yields are based on private communications, as acknowledged in footnotes 74 and 84; it would improve reproducibility to give the resulting yields with stated uncertainties or to cite public sources where available.
  5. [Eq. (13)] The flat distribution in E_N follows from isotropy in the parent-meson rest frame; the text should state this assumption explicitly and note that angular correlations in the three-body decay are neglected.

Circularity Check

0 steps flagged · score 0.0 of 10

No circularity found: the sensitivity reaches are computed from standard model amplitudes, an explicit decay-width calculation, and external experimental event-number inputs.

full rationale

The derivation chain is self-contained: (i) the LNV amplitude in Eq. (4) follows from the standard charged-current Lagrangian; (ii) the total width in Eq. (7) is computed in the Appendix from explicit tree-level partial widths; (iii) event counts are built from Eqs. (12)-(13) using the decay probability in Eq. (8); (iv) limits are obtained by equating the event count to the external Feldman-Cousins benchmark of 3.09 events at 95% C.L. In each step, the mixing V_lN is the unknown parameter being solved for, not a fitted input renamed as a prediction. The democratic choice |V_eN|^2=|V_muN|^2=|V_tauN|^2 in the total width is an explicitly stated assumption, and it approximately cancels in the branching-ratio factor in the short-decay-length regime where the SHiP and NA62 reach is quoted. The paper's many self-citations are contextual and do not carry the central velocity-effect comparison. The Sec. 8 caveat about idealized 100% detection/reconstruction efficiency and the separate geometric-baseline concern about omitting exp(-L_in/L_N) are quantitative modeling limitations, not circular reductions, because the same incomplete probability is used as an input in both the at-rest and in-flight comparisons rather than being derived from the quantity the paper claims to predict.

Assumptions & free parameters 2 free parameters · 8 assumptions · 1 invented entities

The central claim rests on the standard seesaw extension, boosted decay kinematics, and a set of experimental input numbers. The only hand-set scalar values are the democratic mixing benchmark and the zero-background assumption. The scanned heavy neutrino mass and mixing angles are the independent variables, not fitted parameters.

free parameters (2)
  • Democratic mixing scale in Gamma_N = |VeN|^2=|VmuN|^2=|VtauN|^2=1
    Sec. 6 sets all three active-sterile mixings equal when computing the heavy neutrino total width used in the sensitivity bounds. The quoted reaches depend on this normalization choice.
  • Background event count = 0
    Sec. 6 assumes zero background events and uses 3.09 signal events at 95% C.L. as the upper limit. Nonzero backgrounds would weaken every quoted sensitivity curve.
assumptions (8)
  • domain assumption One light right-handed Majorana neutrino in the 0.14 to 6 GeV mass range mediates the LNV decays, with two heavier right-handed neutrinos assumed irrelevant.
    Sec. 2 introduces the single light RH neutrino and defers neutrino mass generation to two heavier states.
  • standard math Narrow-width approximation for the on-shell heavy neutrino.
    Sec. 4 uses Gamma_N much smaller than M_N to factor the three-body rate into a production branching fraction times a decay branching fraction.
  • domain assumption Light neutrino exchange in the LNV meson decays is negligible.
    Sec. 3 states that non-resonant light neutrino contributions are negligibly small.
  • ad hoc to paper Democratic mixing normalization in Gamma_N when deriving limits.
    Sec. 6 assumes |VeN|^2=|VmuN|^2=|VtauN|^2 when computing the total width, which is a benchmark choice rather than a model prediction.
  • ad hoc to paper Zero background and 100 percent detection and reconstruction efficiency.
    Sec. 6 assumes zero background events and Sec. 8 states the detector is idealized with 100 percent efficiency. This directly sets the scale of the claimed reaches.
  • ad hoc to paper Detector geometry is a single length with no angular acceptance.
    Eq. 8 and Eq. 13 use one length LD and only require that N decays within it, ignoring geometric acceptance and vertex reconstruction cuts.
  • domain assumption Fixed parent meson momenta for SHiP, NA62, and FCC-ee.
    Sec. 7 assigns single momenta such as pK=75 GeV and pDs=58 GeV instead of integrating over the full meson momentum spectrum.
  • domain assumption Meson production yields are taken from experimental projections and private communications without error bars.
    Sec. 7 lists N_meson values for LHCb, SHiP, MATHUSLA, Belle II, and FCC-ee; uncertainties on these yields are not propagated.
invented entities (1)
  • Right-handed Majorana neutrino N independent evidence
    purpose: Mediates the LNV meson decays M1- -> l1- l2- pi+ through on-shell production and decay.
    The particle is not new to this paper, but it is introduced in Sec. 2 as the BSM state. It has falsifiable handles in displaced vertex searches, peak searches, and LNV meson decays.

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Cite this review

Pith. "Pith review of Sensitivity of Lepton Number Violating Meson Decays in Different Experiments." pith.science (2026). https://pith.science/paper/7CNLU3DH

@misc{pith2026190809562,
  author       = {Pith},
  title        = {Pith review of: Sensitivity of Lepton Number Violating Meson Decays in Different Experiments},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/7CNLU3DH}},
  note         = {Machine review of arXiv:1908.09562}
}
abstract

We study the discovery prospect of different three body lepton number violating~(LNV) meson decays $M_{1}^{-}\to\ell_{1}^{-}\ell_{2}^{-}M_{2}^{+}$ in the framework of right handed~(RH) neutrino extended Standard Model~(SM). We consider a number of ongoing experiments, such as, NA62 and LHCb at CERN, Belle II at SuperKEK, as well as at the proposed future experiments, SHiP, MATHUSLA and FCC-ee. The RH Majorana neutrino $N$ mediating these meson decays provides a resonant enhancement of the rates, if the mass of $N$ lies in the range $(100\, \text{MeV}-6\, \text{GeV})$. We consider the effect of parent mesons velocity, as well as, the effect of finite detector size. Using the expected upper limits on the number of events for the LNV decay modes, $M_{1}^{-} \to\ell_1^{-}\ell_2^{-}\pi^{+}$~($M_{1}=B, B_c,D, D_{s}\,\text{and}\,K$), we analyze the sensitivity reach of the mixing angles $|V_{e N}|^{2}$, $|V_{\mu N}|^{2}$, $|V_{\tau N}|^{2}$, $|V_{e N}V_{\mu N}|$, $|V_{e N}V_{\tau N}|$ and $|V_{\mu N}V_{\tau N}|$ as a function of heavy neutrino mass $M_{N}$. We show that, inclusion of parent meson velocity can account to a large difference for active-sterile mixing, specially for $D$, $D_s$ meson decay at SHiP and $K$ meson decay at NA62. Taking into account the velocity of the $D_s$ meson, the future beam dump experiment SHiP can probe $|V_{eN}|^2 \sim 10^{-9}$. For RH neutrino mass in between 2 - 5 GeV, MATHUSLA can provide best sensitivity reach of active-sterile mixings.

Figures

Figures reproduced from arXiv: 1908.09562 by the authors.

Figure 1
Figure 1. FIG. 1. The Feynman diagrams for the lepton number violating meson decays. These processes [PITH_FULL_IMAGE:figures/full_fig_p005_1.png] view at source ↗
Figure 2
Figure 2. FIG. 2. The t-channel diagram for the lepton number violating meson decay. See text for details. [PITH_FULL_IMAGE:figures/full_fig_p005_2.png] view at source ↗
Figure 3
Figure 3. FIG. 3. Left panel: The total decay width of heavy neutrino N with the assumption, [PITH_FULL_IMAGE:figures/full_fig_p007_3.png] view at source ↗
Figures from the paper (13 more)
Figure 4
Figure 4. Figure 4: FIG. 4. Dependence of the decay length [PITH_FULL_IMAGE:figures/full_fig_p008_4.png]
Figure 5
Figure 5. Figure 5: FIG. 5. Dependence of parameter [PITH_FULL_IMAGE:figures/full_fig_p009_5.png]
Figure 6
Figure 6. Figure 6: FIG. 6. Future sensitivity reach and present limits on the mixing angles as a function of RH [PITH_FULL_IMAGE:figures/full_fig_p015_6.png]
Figure 7
Figure 7. Figure 7: FIG. 7. Variation of the future sensitivity reach and present limits on the mixing angle [PITH_FULL_IMAGE:figures/full_fig_p016_7.png]
Figure 8
Figure 8. Figure 8: FIG. 8. Same as Fig [PITH_FULL_IMAGE:figures/full_fig_p017_8.png]
Figure 9
Figure 9. Figure 9: FIG. 9. Same as Fig [PITH_FULL_IMAGE:figures/full_fig_p018_9.png]
Figure 10
Figure 10. Figure 10: FIG. 10. Future sensitivity reach and present limits on the mixing angles [PITH_FULL_IMAGE:figures/full_fig_p019_10.png]
Figure 11
Figure 11. Figure 11: FIG. 11. Combined bounds (thick-blue solid) on mixing angle [PITH_FULL_IMAGE:figures/full_fig_p021_11.png]
Figure 12
Figure 12. Figure 12: FIG. 12. Combined bounds (thick-blue solid) on mixing angle [PITH_FULL_IMAGE:figures/full_fig_p022_12.png]
Figure 13
Figure 13. Figure 13: FIG. 13. Combined bounds (thick-blue solid) on mixing angle [PITH_FULL_IMAGE:figures/full_fig_p023_13.png]
Figure 14
Figure 14. Figure 14: FIG. 14. Combined bounds on mixing angle [PITH_FULL_IMAGE:figures/full_fig_p025_14.png]
Figure 15
Figure 15. Figure 15: FIG. 15. Combined bounds (thick-blue solid) on mixing angle [PITH_FULL_IMAGE:figures/full_fig_p025_15.png]
Figure 16
Figure 16. Figure 16: FIG. 16. Combined bounds (thick-blue solid) on mixing angle [PITH_FULL_IMAGE:figures/full_fig_p026_16.png]

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Reference graph

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