Probing a 146 GeV cLFV scalar using the LHC and low-energy experiments

Christina Gao; Lingfeng Li; Z.J. Xiong

REVIEW 2 major objections 5 minor 51 references

A 146 GeV scalar that could explain the CMS eμ excess is already under pressure from μ–e conversion and will be settled by low-energy experiments within a decade.

Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →

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T0 review · grok-4.5

2026-07-12 03:48 UTC pith:26FNSSNM

load-bearing objection Clean model-agnostic MCMC of a 146 GeV cLFV scalar; the non-zero Y_eμ peak is real but carries only ~20% posterior weight under a 2.8σ Gaussian pull, so the abstract overstates preference while the complementarity map remains useful. the 2 major comments →

arxiv 2607.03249 v1 pith:26FNSSNM submitted 2026-07-03 hep-ph hep-th

Probing a 146 GeV cLFV scalar using the LHC and low-energy experiments

Christina Gao , Lingfeng Li , Z.J. Xiong This is my paper

classification hep-ph hep-th

keywords charged lepton flavor violation146 GeV scalarCMS eμ excessμ–e conversionBayesian MCMCeffective LagrangianMu2eCOMET

verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved

The pith

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

CMS has reported a 2.8σ excess consistent with a 146 GeV scalar decaying to an electron-muon pair. The paper asks whether any such scalar can also survive the full suite of existing low-energy charged-lepton-flavor-violation bounds. It introduces a minimal effective description with seven free couplings (one gluonic, six Yukawa) that control both the LHC rate and every low-energy process, then maps the allowed region with a Bayesian MCMC scan. The posterior is bimodal: one mode clusters near vanishing flavor-violating couplings, the other peaks at Yeμ ≈ 10^{-4.09} and is already grazed by present μ–e conversion limits. Projected reaches of Mu2e, COMET, Mu3e, MEG II, Belle II, STCF and the HL-LHC cover essentially the entire non-zero mode, so the scalar interpretation of the excess will be confirmed or excluded within roughly ten years.

Core claim

A single real scalar of mass 146 GeV coupled to gluons and to all charged-lepton bilinears yields a preferred posterior mode with Yeμ ∼ 10^{-4.09} that simultaneously accommodates the CMS eμ excess and existing cLFV null results; that mode is already cut by current μ–e conversion and lies fully within the projected sensitivity of the next generation of low-energy experiments.

What carries the argument

The seven-parameter effective Lagrangian of Eq. (1.1)/(2.1) together with a Bayesian MCMC likelihood that folds the CMS 3.89 fb excess, same-flavor di-lepton resonance limits, and the complete set of low-energy cLFV branching-ratio bounds into a single posterior.

Load-bearing premise

Treating the CMS 2.8σ global excess as a Gaussian likelihood centered on 3.89 fb while still allowing a vanishing-coupling mode, so the non-zero peak is only as strong as that modest excess.

What would settle it

A null result from Mu2e or COMET at the projected 10^{-16}–10^{-17} conversion sensitivity would exclude the entire non-zero Yeμ peak preferred by the CMS excess.

Watch this falsifier — get emailed when new claim-graph text bears on it.

If this is right

Mu2e and COMET will cover almost the entire CMS-favored Yeμ island, leaving at most a narrow surviving strip.
Mu3e will independently close the same island through the three-electron channel.
MEG II will tighten the product Yeτ Yμτ by an order of magnitude, further restricting the τ-sector plane.
HL-LHC same-flavor di-lepton searches will push the diagonal Yukawas Ye e, Yμμ, Yττ well below their present loose upper limits.
If the excess is real, a heterogeneous data set from muon experiments, τ factories and the HL-LHC will pin down the seven couplings within a decade.

Where Pith is reading between the lines

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

Because the non-zero mode is already grazed by present conversion limits, even a modest improvement in SINDRUM-II-style data could have shifted the posterior weight decisively toward vanishing couplings.
The same seven-parameter scaffold can be reused for any future resonance claim in a different di-lepton channel without committing to a specific ultraviolet completion.
A parallel lepton-PDF production analysis would test whether the excess can be rescued by a purely leptophilic production mechanism that evades the gluonic conversion bound.

Editorial analysis

A structured set of objections, weighed in public.

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

Desk Editor's Note

Clean model-agnostic MCMC of a 146 GeV cLFV scalar; the non-zero Y_eμ peak is real but carries only ~20% posterior weight under a 2.8σ Gaussian pull, so the abstract overstates preference while the complementarity map remains useful.

read the letter

The new piece is a seven-parameter Bayesian scan of a single real scalar (M=146 GeV, κ_gg + all lepton Yukawas) that folds the CMS eμ excess together with the full low-energy cLFV suite and same-flavor LHC searches. Earlier papers were UV-specific; this one is deliberately model-agnostic and maps future Mu2e/COMET/Mu3e/MEG-II/Belle-II/HL-LHC reaches onto the same posterior. That is useful and cleanly done.

Production and decay formulae are standard. The K-factor is taken from the heavy-top SM calculation with the residual uncertainty acknowledged; form factors use FLAG 2024; likelihoods for upper limits are ordinary one-sided Gaussians. Appendices check prior dependence, HPD intervals, and Z→ℓαℓβ loops. The math and citation pattern look solid.

The soft spot is the language around the “preferred mode.” They center a Gaussian on σ=3.89 fb with width=3.89/2.8, so χ^{2}_146 actively pulls toward non-zero κ_gg Y_eμ. Section 4.2 itself reports that this mode carries only ~20% of the 68% posterior weight; the dominant mode is the vanishing-coupling plateau favored by the nulls. The peak at log10 Y_eμ≈−4.09 is therefore an output of that likelihood construction, not a robust preference. If the CMS term is dropped or replaced by a one-sided limit, the non-zero peak disappears. The abstract’s wording is therefore stronger than the posterior supports. The complementarity statement for future experiments still holds: those experiments will cover the region the excess would select.

This is for people who work on cLFV phenomenology or who need a ready-made effective-parameter map for the next round of muon and τ experiments. It is not a discovery claim and does not need to be. I would send it to referees; the calculation is transparent and the mapping is worth having. Engage if you are writing on the same excess or on low-energy reach; otherwise it is optional reading.

Referee Report

2 major / 5 minor

Summary. The paper studies whether a single real scalar of mass 146 GeV, coupled to gluons via a dimension-5 operator and to all charged-lepton bilinears via seven real Yukawa couplings (Eqs. 1.1/2.1), can accommodate the CMS local excess in pp oϕ o eμ while remaining compatible with the full suite of low-energy cLFV bounds. Production is computed in the ggF channel with a heavy-top K-factor (Sec. 3.1); tree-level and one-loop rates for μ−e conversion, muonium oscillation, three-lepton and radiative LFV decays, and semileptonic τ decays are derived in Sec. 3.2. A Bayesian MCMC scan with log-uniform priors (Sec. 4.1) yields a bimodal posterior: a vanishing-coupling mode favored by the null results and a secondary non-zero mode peaking near log10 Yeμ≃−4.09 that is already constrained by current μ−e conversion. Projected sensitivities of Mu2e, COMET, Mu3e, MEG II, Belle II, STCF and the HL-LHC are shown to cover the CMS-selected region, establishing quantitative complementarity between high- and low-energy probes.

Significance. If the CMS excess is real, a model-agnostic map of the seven-parameter space that simultaneously satisfies LHC production and every major low-energy cLFV observable is a useful and timely contribution. The paper supplies explicit, standard formulae for all rates, uses FLAG 2024 form factors, and produces falsifiable projections that future experiments can test within a decade. The bimodal posterior itself is an informative result: it quantifies how weakly a 2.8σ global excess pulls against existing nulls. The analysis is therefore valuable both as a consistency check of the scalar interpretation and as a concrete target list for Mu2e/COMET/Mu3e and HL-LHC.

major comments (2)

Abstract and Sec. 4.2: the language “preferred mode with peaked value Yeμ∼10−4.09” overstates the posterior. Sec. 4.2 itself states that the vanishing-coupling mode carries roughly 80 % of the 68 % posterior weight; the non-zero peak is secondary. Because χ^{2}_146 (Eqs. 4.3–4.4) converts a 2.8σ global excess into a two-sided Gaussian centered on 3.89 fb, the non-zero mode is an artifact of that likelihood construction. The abstract and conclusions should be rewritten to state that the posterior is bimodal, that the dominant mode is vanishing couplings, and that the non-zero peak is only a secondary feature pulled by the modest CMS excess.
Sec. 4.1, Eqs. (4.3)–(4.4): the treatment of the CMS excess is inconsistent with the treatment of the same-flavor resonance searches (Eq. 4.5). The latter correctly use one-sided upper-limit χ^{2}; the former actively pulls toward a non-zero signal. Given that the global significance is only 2.8σ and the look-elsewhere effect has already been folded in, a more conservative construction (one-sided upper limit, or a mixture that includes the background-only hypothesis) should be shown as a robustness check. If that check eliminates the non-zero peak, the “preferred Yeμ” claim must be dropped.

minor comments (5)

Sec. 3.2.3, Eqs. (3.32)–(3.33): the interference terms in τ→eeμ and τ→μμe are dropped “for simplicity.” A short numerical statement that their inclusion does not shift the posterior would strengthen the claim.
App. B and Fig. 9: the prior-sensitivity test is useful; it would help the reader if the same exercise were repeated for the two-dimensional credible regions of (κgg,Yeμ) that drive the complementarity argument.
Fig. 5 caption and Table 2: the 68 % HPD intervals for log10 Yeμ are reported as two disjoint intervals, yet the abstract quotes only the non-zero peak. Align the abstract wording with the table.
Sec. 3.1: the residual model dependence of the SM K-factor for a pure dimension-5 ϕGG operator is stated to be “percent-level”; a one-sentence reference to the finite-mt literature would make the claim fully transparent.
Typos / notation: “T oy Model” in the contents; occasional missing spaces around ∼ and in “µ−econversion”; “Brth” vs. “Brlim” notation is clear but could be standardized in the tables.

Circularity Check

0 steps flagged

No significant circularity: free parameters scanned against external experimental inputs; posterior peak is an output of the fit, not an input renamed as prediction.

full rationale

The seven couplings are free parameters with log-uniform priors. All observables (ggF cross section via κ_gg and Br(φ o eμ), μ−e conversion, muonium oscillation, three-lepton and radiative decays, τ semi-leptonic modes, same-flavor LHC resonance limits) are computed from the Lagrangian and compared to external experimental numbers (CMS 3.89 fb excess, SINDRUM II, MEG II, Belle, etc.). The MCMC posterior, including the bimodal structure and the non-zero peak at log10(Y_eμ)∼−4.09, is therefore a genuine output of that multi-experiment likelihood, not a quantity forced by construction from a fitted constant or a self-citation. The paper itself reports that the vanishing-coupling mode carries the larger posterior weight and that the CMS global significance is only 2.8σ; no equation equates a claimed prediction to an input by definition. Standard external lattice/PDF/K-factor inputs are used without load-bearing self-citation chains. Methodological choices about how to encode a modest excess as a Gaussian pull affect the strength of the non-zero mode but do not constitute circularity under the defined patterns.

Axiom & Free-Parameter Ledger

3 free parameters · 5 axioms · 1 invented entities

The central claim rests on seven free Yukawa/gluon couplings scanned with log-uniform priors, the assumption that a single real CP-even scalar of fixed mass 146 GeV mediates all processes via the written operators, and standard narrow-width and heavy-top approximations. No new conserved charges or extra dimensions are introduced; the scalar itself is the sole invented entity and is tied directly to the CMS mass peak.

free parameters (3)

κ_gg
Dimensionful gluon coupling controlling ggF production; scanned log-uniform over ten orders of magnitude and fitted to the CMS excess plus low-energy bounds.
Y_eμ
Off-diagonal Yukawa fixed by the eμ excess and constrained by μ−e conversion and muonium oscillation; preferred posterior peak is the main result.
Y_eτ, Y_μτ, Y_ee, Y_μμ, Y_ττ
Remaining five Yukawas enter total width, radiative and three-body decays; only 95 % upper limits are reported because they are not directly fixed by the CMS excess.

axioms (5)

domain assumption All seven couplings are real, Y_ij = Y_ji, and ϕ is CP-even with no ϕG̃G operator.
Stated in Sec. 2; simplifies the parameter space and eliminates CP-odd observables.
domain assumption ϕ is an on-shell resonance treated in the narrow-width approximation.
Used throughout Sec. 3.1 for the production×branching formula.
domain assumption The K-factor for gg→ϕ is identical to the SM heavy-top Higgs K-factor (K≈1.47).
Adopted in Eq. (3.7) with residual finite-m_t uncertainty argued to be percent-level.
ad hoc to paper Log-uniform priors on all |Y_ij| and κ_gg spanning [10^{-10},1].
Chosen in Sec. 4.1 for maximal ignorance; one-sided limits retain mild prior dependence quantified in App. B.
domain assumption Likelihoods for experimental upper limits are one-sided Gaussians (1.645σ for 90 % CL, 1.96σ for 95 % CL).
Standard approximation used in Eqs. (4.5)–(4.6).

invented entities (1)

Real scalar ϕ of mass exactly 146 GeV with the seven free couplings of Eq. (1.1)/(2.1) no independent evidence
purpose: Provide a model-agnostic mediator that can produce the CMS eμ excess while being constrained by all low-energy cLFV observables.
Mass is taken from the CMS peak; couplings are free parameters rather than derived from a UV completion. Independent evidence would be confirmation of the excess or observation of the predicted low-energy rates.

pith-pipeline@v1.1.0-grok45 · 24337 in / 3180 out tokens · 31083 ms · 2026-07-12T03:48:32.825377+00:00 · methodology

0 comments

read the original abstract

The CMS Collaboration reported a local excess at $146~\mathrm{GeV}$ in the search for the lepton-flavor-violating decay of the Higgs boson and additional Higgs bosons in the $e\mu$ final state at $\sqrt{s}= 13~\mathrm{TeV}$. If confirmed, this would constitute a major piece of evidence of charged lepton flavor violation (cLFV). We investigate the compatibility of the claimed signal with the full suite of existing low-energy cLFV constraints in a bottom-up effective description: a single real scalar of mass $146~\mathrm{GeV}$ coupled to gluons and to all charged-lepton bilinears, with seven free parameters that simultaneously control the LHC production cross section, every di-lepton decay channel, and every low-energy cLFV observable. A Bayesian MCMC analysis against $\mu-e$ conversion, muonium-antimuonium oscillation, three-lepton and radiative LFV decays, semileptonic $\tau$ LFV decays, and LHC di-lepton searches yields a preferred mode with peaked value $Y_{e\mu} \sim 10^{-4.09}$, already cut into by the current $\mu-e$ conversion limits. The projected sensitivities of Mu2e, COMET, Mu3e, MACE, MEG~II, Belle~II, STCF, and the HL-LHC directly probe the region of coupling space selected by the CMS excess, so the complementarity between high-energy and low-energy cLFV probes will either corroborate or decisively exclude the scalar interpretation of the anomaly within the next decade.

discussion (0)

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