Expanding the landscape of exotic muon decays

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 → 5 e , μ → 7 e , 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 axionlike 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.