Lepton Flavor Violation with Displaced Vertices

Abstract

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

Comments: 7 pages, to appear in PLB