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Expanding the Landscape of Exotic Muon Decays

Admir Greljo, Ajdin Palavrić, Mirsad Tunja, Jure Zupan

TL;DR

This work develops a signature-driven EFT framework to explore exotic muon decays with high multiplicity, such as $oldsymbol{ ext{μ→7e}}$ or $oldsymbol{ ext{μ→5e}}$, using SMEFT for heavy NP and SMEFT$_X$ for light SM-singlet states. It shows that high-multiplicity channels can access UV scales far beyond traditional SMEFT reach when light states participate, and it provides four concrete benchmark models (flavor-protected scalar, dark photon with LFV, thermal inelastic DM, and on-/off-shell ALP) to illustrate the phenomenology and guide experimental searches at MEG II, Mu3e, COMET, and Mu2e. The analysis clarifies how operator dimension, derivative couplings, and LFV flavor structures shape which signatures dominate in different regions of parameter space. Overall, the work highlights the complementarity between high-precision muon experiments and dark-sector searches in uncovering short-distance lepton-flavor dynamics and the pattern of symmetry breaking in the lepton sector.

Abstract

We chart new-physics models that produce exotic, high-multiplicity muon decays featuring prompt or displaced $e^+e^-$ pairs and/or photons, with or without missing energy, such as $μ\to 5e$, $μ\to 7e$, etc. Starting from an effective-field-theory perspective, we estimate the reach on the ultraviolet scale and identify conditions under which lower-multiplicity modes are suppressed or occur at comparable rates. We then construct explicit realizations in minimal dark-sector models with light, feebly interacting particles, such as flavor-protected scalars, dark photons, inelastic dark matter, and axion-like particles. The predicted novel signatures can be probed at MEG II and Mu3e, as well as during calibration runs of COMET and Mu2e. A future discovery would provide valuable insights into short-distance dynamics and the mechanism of lepton-flavor symmetry breaking.

Expanding the Landscape of Exotic Muon Decays

TL;DR

This work develops a signature-driven EFT framework to explore exotic muon decays with high multiplicity, such as or , using SMEFT for heavy NP and SMEFT for light SM-singlet states. It shows that high-multiplicity channels can access UV scales far beyond traditional SMEFT reach when light states participate, and it provides four concrete benchmark models (flavor-protected scalar, dark photon with LFV, thermal inelastic DM, and on-/off-shell ALP) to illustrate the phenomenology and guide experimental searches at MEG II, Mu3e, COMET, and Mu2e. The analysis clarifies how operator dimension, derivative couplings, and LFV flavor structures shape which signatures dominate in different regions of parameter space. Overall, the work highlights the complementarity between high-precision muon experiments and dark-sector searches in uncovering short-distance lepton-flavor dynamics and the pattern of symmetry breaking in the lepton sector.

Abstract

We chart new-physics models that produce exotic, high-multiplicity muon decays featuring prompt or displaced pairs and/or photons, with or without missing energy, such as , , etc. Starting from an effective-field-theory perspective, we estimate the reach on the ultraviolet scale and identify conditions under which lower-multiplicity modes are suppressed or occur at comparable rates. We then construct explicit realizations in minimal dark-sector models with light, feebly interacting particles, such as flavor-protected scalars, dark photons, inelastic dark matter, and axion-like particles. The predicted novel signatures can be probed at MEG II and Mu3e, as well as during calibration runs of COMET and Mu2e. A future discovery would provide valuable insights into short-distance dynamics and the mechanism of lepton-flavor symmetry breaking.

Paper Structure

This paper contains 12 sections, 69 equations, 10 figures, 4 tables.

Table of Contents

  1. Introduction
  2. Effective Field Theory Considerations
  3. Standard Model Effective Field Theory
  4. Benchmark New Physics Models
  5. Model I: Flavor-Protected Scalar
  6. Model II: Dark Photon with Flavor Violation
  7. Model III: Thermal Inelastic Dark Matter
  8. Model IV: On- and Off-Shell ALP
  9. Conclusions
  10. Dark sectors gauged under $U(1)_{\mu-e}$
  11. Gauged $U(1)_{\mu-e}$ model
  12. Inelastic thermal DM and $U(1)_{\mu-e}$

Figures (10)

  • Figure 1: Sensitivity to the effective UV scale $\Lambda$ in \ref{['eq:opsModelI']}, setting ${\mathcal{C}}_{\mathcal{S}}^{(3)}=y_\mu$ from decay $\mu \to e + 3\,\mathcal{S}$, followed by $\mathcal{S} \to 2e$, yielding a 7-electron final state. The curves show the values of $\Lambda$ that yield $\mathrm{BR} \times \mathrm{acceptance} = 10^{\vcenter{\hbox{[1]{$-$}}}15}$ as a function of the scalar mass $m_\mathcal{S}$, for different kinematical cuts, requiring either 6 or 7 electrons and positrons above the kinematic threshold. The shaded region, $m_{\mathcal{S}}\lesssim10\,\mathrm{MeV}$, is excluded by the searches at beam-dump experiments Heeck:2017xmg.
  • Figure 2: Feynman diagrams generating the two operators in \ref{['eq:opsModelI']} in the flavor-protected scalar model. The red dot indicates the suppression by the small spurion $\varepsilon_\mu$. See \ref{['sec:modelI']} for details.
  • Figure 3: Feynman diagrams corresponding to the cascade transition given by \ref{['eq:DM_cascade_1']}. See \ref{['sec:DarkPhoton']} for details.
  • Figure 4: Feynman diagram giving the cascade decay in \ref{['eq:DM_cascade_2']} for the thermal inelastic dark matter model. See \ref{['sec:model_dark_higgs']} for details.
  • Figure 5: Exclusion contours in the $(m_{\chi_1}, \varepsilon)$ plane for the benchmark scenario defined in \ref{['eq:DM_benchmark']}. The shaded region denotes the constraints from CHARM Duerr:2019dmv, while the solid lines correspond to the projected sensitivity for different values of the cLFV scale $\Lambda$. The dashed line denotes the parameter space compatible with the observed dark matter relic abundance.
  • ...and 5 more figures