Lepton flavor violating Z decays: A promising window to low scale seesaw neutrinos

Understand

In this paper we study the Lepton Flavor Violating $Z$ boson decays $Z \to \tau \mu$ and $Z \to \tau e$ in the context of low scale seesaw models with new heavy Majorana neutrinos whose masses could be reachable at the LHC.

• Our computations of the decay rates are done in the particular realization given by the Inverse Seesaw Model with six extra heavy neutrinos which are quasi-degenerate in three pseudo-Dirac pairs.
• In particular, we focus on scenarios that are built ad-hoc to produce suppressed rates in all the processes involving $\mu$-$e$ transitions, given the fact that these are by far the most strongly constrained by present data.
• We will fully explore the $Z \to \tau \mu$ and $Z \to \tau e$ rates, together with a set of observables that we find to be the most constraining ones, and we will conclude that sizable rates of up to $2 \times 10^{-7}$, accessible at future colliders, can be reached in this model for Majorana masses in the few TeV range, potentially reachable at LHC.