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 ($$ \mathcal{B} $$ B = 4.0 × 10−10) and find that the Mu3e experiment should have tens to hundreds of signal events per 1015μ+ 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 hd. Following its production in μ+ → e+hd decays, the dark Higgs can undergo a decay cascade to two e+e− pairs through two dark photons, hd→ γdγd→ 2(e+e−). We show that a μ+→ 3e+ 2e− search at the Mu3e experiment, with potential sensitivity to the branching ratio at the $$ \mathcal{O} $$ O (10−12) level or below, can explore new regions of parameter space and new physics scales as high as Λ ∼ 1015 GeV.