Lepton flavor violating decay of true muonium: $\boldsymbol{(μ^+ μ^-) \to μ^\pm e^\mp}$

Abstract: We propose a new channel for probing charged lepton flavor violation (CLFV): the decay of true muonium into a lepton pair of different flavor, $(\mu^+ \mu^-) \to \mu^\pm e^\mp$. This purely leptonic two-body decay provides a clean experimental signature in the form of energetic, oppositely charged leptons. It is sensitive not only to photonic dipole interactions but also to four-fermion contact interactions, and is free from hadronic uncertainties in theoretical predictions. We evaluate the branching ratios induced by scalar-, vector-, and dipole-type CLFV operators. Our results show that the branching ratio can reach up to $\mathcal{O}(10^{-20})$ within current experimental bounds. Such a decay may be observable at future high-luminosity muon beamlines or at dedicated $e^+e^-$ or $\mu^+\mu^-$ colliders.