Lepton Flavor Violation with Displaced Vertices

TL;DR

This paper explores how light new physics with lepton-flavor-violating couplings could be detected via displaced vertices, especially in tau decays, offering a new approach beyond traditional photon emission searches.

Contribution

It demonstrates that displaced-vertex searches can significantly improve limits on lepton-flavor violation involving light bosons, particularly in tau decays, and identifies the narrow parameter space for muon decay signatures.

Findings

Displaced vertices are rare in muon decays but more promising in tau decays.

Conventional bounds are conservative; displaced vertices can provide stronger limits.

Tau decay signatures offer richer phenomenology for lepton-flavor violation.

Abstract

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.