Lepton flavor violation with displaced vertices.

If light new physics with lepton-flavor-violating couplings exists, the prime discovery channel might not be ℓ → ℓ ′ γ but rather ℓ → ℓ ′ X , where the new boson X could be an axion, majoron, familon or Z ′ gauge boson. The most conservative bound then comes from ℓ → ℓ ′ + inv , but if the on-shell X can decay back into leptons or photons, displaced-vertex searches could give much better limits. We show that only a narrow region in parameter space allows for displaced vertices in muon decays, μ → e X , X → γ γ , e e , whereas tauon decays can have much more interesting signatures. [ABSTRACT FROM AUTHOR]