New physics in multi-electron muon decays

We study the exotic muon decays with five charged tracks in the final state. First, we investigate the Standard Model rate for μ + → 3 e + 2 e − 2 ν ( ℬ = 4 . 0 × 10 − 10 ) and find that the Mu3e experiment should have tens to hundreds of signal events per 10 15 μ + decays, depending on the signal selection strategy. We then turn to a neutrinoless μ + → 3 e + 2 e − decay that may arise in new-physics models with lepton-flavor-violating effective operators involving a dark Higgs h d . Following its production in μ + → e + h d decays, the dark Higgs can undergo a decay cascade to two e + e − pairs through two dark photons, h d → γ d γ d → 2( e + e − ). We show that a μ + → 3 e + 2 e − search at the Mu3e experiment, with potential sensitivity to the branching ratio at the 𝒪 (10 − 12 ) level or below, can explore new regions of parameter space and new physics scales as high as Λ ∼ 10 15 GeV.