Lepton Flavor Violation with Displaced Vertices

If light new physics with lepton-flavor-violating couplings exists, the prime discovery channel might not be $\ell\to\ell'\gamma$ but rather $\ell\to\ell' X$, where the new boson $X$ could be an axion, majoron, familon or Z' gauge boson. The most conservative bound then comes from $\ell\to\ell'+\mathrm{inv}$, 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, $\mu\to e X, X\to \gamma\gamma, ee$, whereas tauon decays can have much more interesting signatures.