New physics in multi-electron muon decays

Abstract

Abstract

We study the exotic muon decays with five charged tracks in the final state. First, we investigate the Standard Model rate for μ⁺ → 3e⁺ 2e⁻ 2ν (\( \mathcal{B} \) = 4.0 × 10⁻¹⁰) and find that the Mu3e experiment should have tens to hundreds of signal events per 10¹⁵ μ⁺ decays, depending on the signal selection strategy. We then turn to a neutrinoless μ⁺ → 3e⁺ 2e⁻ 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_dγ_d → 2(e⁺e⁻). We show that a μ⁺ → 3e⁺ 2e⁻ search at the Mu3e experiment, with potential sensitivity to the branching ratio at the \( \mathcal{O} \)(10⁻¹²) level or below, can explore new regions of parameter space and new physics scales as high as Λ ∼ 10¹⁵ GeV.