Impact of sterile neutrinos on nuclear-assisted cLFV processes

TL;DR

This paper explores how sterile neutrinos influence charged lepton flavor violation processes in muonic atoms, providing analytical formulas and assessing their potential detectability in current and future experiments.

Contribution

It offers a comprehensive analysis of sterile neutrino effects on cLFV observables, including new analytical expressions and evaluations within different neutrino mass models.

Findings

Sterile neutrinos can significantly enhance cLFV processes.

Some predicted signals are within reach of upcoming experiments.

Constraints from other searches limit sterile neutrino parameter space.

Abstract

We discuss charged lepton flavour violating processes occurring in the presence of muonic atoms, such as muon-electron conversion in nuclei $\text{CR}(\mu -e, \text{ N})$, the (Coulomb enhanced) decay of muonic atoms into a pair of electrons BR($\mu^- e^- \to e^- e^-$, N), as well as Muonium conversion and decay, $\text{Mu}-\bar{\text{Mu}}$ and $\text{Mu}\to e^+ e^-$. Any experimental signal of these observables calls for scenarios of physics beyond the Standard Model. In this work, we consider minimal extensions of the Standard Model via the addition of sterile fermions, providing the corresponding complete analytical expressions for all the considered observables. We first consider an "ad hoc" extension with a single sterile fermion state, and investigate its impact on the above observables. Two well motivated mechanisms of neutrino mass generation are then considered: the Inverse Seesaw embedded into the Standard Model, and the $\nu$MSM. Our study reveals that, depending on their mass range and on the active-sterile mixing angles, sterile neutrinos can give significant contributions to the above mentioned observables, some of them even lying within present and future sensitivity of dedicated cLFV experiments. We complete the analysis by confronting our results to other (direct and indirect) searches for sterile fermions.