arxiv: 2604.13562 · v1 · submitted 2026-04-15 · ✦ hep-ph · hep-ex

Recognition: unknown

Sensitivity to top-quark FCNC interactions at future muon colliders

A. Senol , B. S. Ozaltay , M. Tekin , H. Denizli

Authors on Pith no claims yet

Pith reviewed 2026-05-10 13:32 UTC · model grok-4.3

classification ✦ hep-ph hep-ex

keywords top quarkFCNCmuon collideranomalous couplingseffective field theorybranching ratiosboosted decision treesrare decays

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The pith

A 10 TeV muon collider can probe top-quark FCNC couplings at the O(10^{-3}) level, setting branching ratio limits of O(10^{-6}) that beat current LHC bounds by more than an order of magnitude.

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

This paper investigates how a future muon collider at 10 TeV center-of-mass energy can search for flavor-changing neutral current interactions of the top quark. It focuses on the process mu+ mu- to nu_mu mu+ b j and its charge conjugate, using effective field theory to parametrize anomalous tqZ and tq gamma couplings. Signal and background events are generated with Pythia and Delphes, then separated with a boosted decision tree. With 10 ab^{-1} of data, the projected limits on the couplings reach O(10^{-3}), which translates to branching fraction upper limits around 10^{-6} for the rare top decays. These sensitivities would extend current constraints from CMS and ATLAS by more than a factor of ten and show that a multi-TeV muon collider offers a clean environment for testing rare top-quark processes.

Core claim

The central claim is that the process mu+ mu- to nu_mu mu+ b j at sqrt(s) = 10 TeV, analyzed with a multivariate boosted decision tree after full simulation including parton showering and detector effects, yields projected sensitivities of O(10^{-3}) on the effective couplings kappa_tqZ and lambda_tq gamma. These limits correspond to branching ratio bounds of O(10^{-6}) for t to qZ and t to q gamma decays and improve existing LHC constraints by more than one order of magnitude at an integrated luminosity of 10 ab^{-1}.

What carries the argument

The boosted decision tree discrimination applied to Monte Carlo events for the mu+ mu- to nu_mu mu+ b j final state, used to extract limits on the effective field theory parameters kappa_tqZ and lambda_tq gamma.

If this is right

The muon collider would set branching ratio limits of O(10^{-6}) on t to qZ and t to q gamma decays.
These limits would extend current CMS and ATLAS bounds by more than an order of magnitude.
A multi-TeV muon collider would serve as a powerful and complementary platform for studying rare top-quark interactions.
It would provide a unique opportunity to explore physics beyond the Standard Model through FCNC processes.

Where Pith is reading between the lines

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

The lepton-collider environment suppresses QCD backgrounds that hinder similar searches at hadron colliders, allowing cleaner extraction of rare top decays.
Higher integrated luminosities at the same energy would tighten the coupling limits further without changing the analysis strategy.
The results could be reinterpreted in specific new-physics models that predict particular patterns of top FCNC strengths.

Load-bearing premise

The Monte Carlo event generation, fast detector simulation, and boosted decision tree analysis accurately capture all relevant physics and detector effects without large unmodeled systematic uncertainties.

What would settle it

An observed event yield in the signal region after BDT selection that is inconsistent with the background-only prediction at the level required to reach the projected O(10^{-3}) coupling sensitivity would either support or rule out the anomalous couplings at the claimed precision.

Figures

Figures reproduced from arXiv: 2604.13562 by A. Senol, B. S. Ozaltay, H. Denizli, M. Tekin.

**Figure 2.** Figure 2: FIG. 2: The total cross section ( [PITH_FULL_IMAGE:figures/full_fig_p006_2.png] view at source ↗

**Figure 3.** Figure 3: FIG. 3: The upper panels show the BDT output for the signal sample ( [PITH_FULL_IMAGE:figures/full_fig_p012_3.png] view at source ↗

**Figure 4.** Figure 4: FIG. 4: Normalized distribution of the reconstructed invariant mass [PITH_FULL_IMAGE:figures/full_fig_p014_4.png] view at source ↗

**Figure 5.** Figure 5: FIG. 5: Statistical significance (SS) as a function of the anomalous couplings [PITH_FULL_IMAGE:figures/full_fig_p016_5.png] view at source ↗

**Figure 6.** Figure 6: FIG. 6: Projected sensitivity in the BR( [PITH_FULL_IMAGE:figures/full_fig_p020_6.png] view at source ↗

read the original abstract

We investigate flavor-changing neutral current (FCNC) interactions of the top quark at a future muon collider with a center-of-mass energy of $\sqrt{s} = 10~\mathrm{TeV}$. The process $\mu^{+}\mu^{-} \to \nu_{\mu}\,\mu^+\,b\,j$ and its corresponding charge conjugate are considered as a probe of anomalous $tqZ$ and $tq\gamma$ couplings, parametrized within an effective field theory framework in terms of $\kappa_{tqZ}$ and $\lambda_{tq\gamma}$. Signal and background events are simulated using Monte Carlo techniques, including parton showering and hadronization with \texttt{Pythia} and a fast detector simulation based on \texttt{Delphes} with a dedicated 10~TeV muon collider setup. A multivariate analysis based on boosted decision trees is employed to enhance the signal discrimination. Assuming an integrated luminosity of $10~\mathrm{ab}^{-1}$, we obtain projected sensitivities to the anomalous couplings at the $\mathcal{O}(10^{-3})$ level, corresponding to branching ratio limits of $\mathcal{O}(10^{-6})$ for the rare $t \to qZ$ and $t \to q\gamma$ decays. These results significantly improve upon the current bounds from the CMS and ATLAS collaborations, extending the sensitivity by more than one order of magnitude. Our findings demonstrate that a multi-TeV muon collider provides a powerful and complementary platform for probing rare top-quark interactions, offering a unique opportunity to explore physics beyond the Standard Model through FCNC processes.

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 concrete projected limits on top FCNC couplings at a 10 TeV muon collider that beat current LHC bounds, but those limits rest on an unvalidated Delphes model for a detector that does not exist.

read the letter

This paper projects how well a 10 TeV muon collider could constrain top-quark flavor-changing neutral currents through the process mu+ mu- to nu_mu mu+ b j and its charge conjugate. They parametrize the interactions with kappa_tqZ and lambda_tq gamma, run signal and background Monte Carlo with Pythia plus a custom Delphes card, and apply a boosted decision tree to extract limits. With 10 ab^{-1} they claim couplings at the 10^{-3} level and branching ratios near 10^{-6}, more than an order of magnitude beyond ATLAS and CMS results. That numerical projection is the concrete new piece; earlier work has done similar sensitivity studies at other machines, but this combination of energy, final state, and multivariate method for a muon collider is not already in the literature. The simulation pipeline itself follows standard practice and the comparison to existing bounds is direct and useful. The soft spot is the detector modeling. Delphes parameters for muon identification, b-tagging, and jet resolution at multi-TeV energies are assumptions with no real data to anchor them. The stress-test concern holds: small shifts in those parameters can move the extracted limits by a factor of several, yet the work does not appear to include dedicated variations or a full GEANT4 cross-check. Background rates and efficiencies therefore carry unquantified uncertainty. The paper is aimed at people planning muon collider experiments or studying top FCNC in effective theories. A reader in that niche can use the numbers as a benchmark, but would treat the exact reach with caution until the detector assumptions are stress-tested. I would send it for peer review. The method is transparent enough that referees can ask for the missing robustness checks and the authors can add them without starting over.

Referee Report

1 major / 0 minor

Summary. The manuscript investigates flavor-changing neutral current (FCNC) interactions of the top quark at a proposed 10 TeV muon collider. It considers the process μ⁺μ⁻ → ν_μ μ⁺ b j (and charge conjugate) as a probe of anomalous tqZ and tqγ couplings parametrized by κ_tqZ and λ_tqγ in an EFT framework. Signal and background events are generated with Pythia, passed through a dedicated Delphes fast simulation for a 10 TeV muon collider detector, and analyzed with boosted decision trees to enhance discrimination. With an assumed integrated luminosity of 10 ab^{-1}, the paper projects sensitivities at the O(10^{-3}) level for the couplings, corresponding to branching-ratio limits of O(10^{-6}) for t → qZ and t → qγ, which would improve on existing CMS and ATLAS bounds by more than an order of magnitude.

Significance. If the simulation chain accurately reproduces efficiencies, background rejection, and kinematic distributions, the projected limits would constitute a meaningful advance over current LHC constraints on top-quark FCNC processes. The study employs standard Monte Carlo tools (Pythia + Delphes) and a multivariate technique that is conventional for such projection studies, thereby providing a clear benchmark for the physics reach of a multi-TeV muon collider in the top sector.

major comments (1)

The central claim of O(10^{-3}) sensitivity (abstract and results) rests on the accuracy of the Pythia+Delphes+BDT pipeline for the μ⁺μ⁻ → ν_μ μ⁺ b j final state. The Delphes card encodes assumed resolutions, efficiencies, and beam-induced backgrounds for a hypothetical 10 TeV muon collider that cannot be calibrated against data. No dedicated variation of muon ID, b-tagging, or jet energy scale is reported to quantify how such changes propagate into the BDT performance and extracted limits. Because even modest shifts in these parameters can alter the projected reach by an order of magnitude, the robustness of the quoted sensitivities cannot be assessed from the presented material.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the careful reading of our manuscript and the positive assessment of its significance. We address the major comment below and commit to revisions that will strengthen the robustness discussion of our projections.

read point-by-point responses

Referee: The central claim of O(10^{-3}) sensitivity (abstract and results) rests on the accuracy of the Pythia+Delphes+BDT pipeline for the μ⁺μ⁻ → ν_μ μ⁺ b j final state. The Delphes card encodes assumed resolutions, efficiencies, and beam-induced backgrounds for a hypothetical 10 TeV muon collider that cannot be calibrated against data. No dedicated variation of muon ID, b-tagging, or jet energy scale is reported to quantify how such changes propagate into the BDT performance and extracted limits. Because even modest shifts in these parameters can alter the projected reach by an order of magnitude, the robustness of the quoted sensitivities cannot be assessed from the presented material.

Authors: We agree that the robustness of the projected sensitivities merits explicit quantification, given that the study relies on a fast simulation with a dedicated Delphes card for a hypothetical detector. The original manuscript did not report dedicated variations of muon identification efficiency, b-tagging efficiency, or jet energy scale and their propagation into the BDT performance and limits. To address this point, the revised manuscript will include a new subsection presenting additional studies in which these parameters are varied within conservative ranges informed by LHC extrapolations. The impact on signal efficiency, background rejection, and the resulting coupling limits will be quantified and discussed. This addition will allow a clearer evaluation of the stability of the O(10^{-3}) reach. revision: yes

Circularity Check

0 steps flagged

No circularity in projected FCNC sensitivities

full rationale

The paper performs standard Monte Carlo simulations of signal and background processes for a hypothetical 10 TeV muon collider using Pythia and Delphes, followed by BDT discrimination to extract projected limits on EFT couplings at assumed luminosity. No load-bearing step reduces by construction to its own inputs: there are no self-definitional equations, no parameters fitted to a data subset and then relabeled as predictions, and no uniqueness theorems or ansatze imported via self-citation. The derivation chain consists of forward-looking estimates dependent on modeling assumptions, but remains independent of the target results and self-contained against external benchmarks such as existing LHC bounds.

Axiom & Free-Parameter Ledger

2 free parameters · 2 axioms · 0 invented entities

The central projection rests on the validity of the EFT parametrization of the couplings and the fidelity of the simulation chain for a hypothetical collider; no new entities are introduced.

free parameters (2)

center-of-mass energy = 10 TeV
Chosen by hand for the future collider scenario the projection assumes.
integrated luminosity = 10 ab^{-1}
Chosen by hand to set the scale of the projected sensitivity.

axioms (2)

domain assumption The effective field theory framework with kappa_tqZ and lambda_tqgamma accurately captures the anomalous couplings up to the energies considered.
Invoked to define the signal process and interpret the limits.
domain assumption Pythia parton showering, hadronization, and the Delphes fast simulation with the dedicated 10 TeV muon collider card faithfully model signal and background at this energy.
Basis for all event generation and detector response used in the BDT.

pith-pipeline@v0.9.0 · 5592 in / 1599 out tokens · 68014 ms · 2026-05-10T13:32:38.377823+00:00 · methodology

discussion (0)

Reference graph

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