Indirect searches for sterile neutrinos at a high-luminosity Z-factory

TL;DR

This paper investigates how sterile neutrinos could induce lepton flavor violating Z decays detectable at future high-luminosity Z-factories like FCC-ee, complementing low-energy experiments and exploring new physics scenarios.

Contribution

It analyzes sterile neutrino effects on Z decays within minimal models and neutrino mass frameworks, highlighting potential observability at FCC-ee.

Findings

Sterile neutrinos can produce detectable BR(Z→ℓ₁ℓ₂) at FCC-ee.

High-luminosity Z-factories complement low-energy LFV searches.

Sterile neutrino contributions are within experimental reach.

Abstract

A future high-luminosity $Z$-factory will offer the possibility to study rare $Z$ decays, as those leading to lepton flavour violating final states. Processes such as $Z \to \ell_1^\mp \ell_2^\pm$ are potentially complementary to low-energy (high-intensity) observables of lepton flavour violation. In this work we address the impact of new sterile fermions on lepton flavour violating $Z$ decays, focusing on potential searches at FCC-ee (TLEP), and taking into account experimental and observational constraints on the sterile states. We consider a minimal extension of the Standard Model by one sterile fermion state, and two well-motivated frameworks of neutrino mass generation, the Inverse Seesaw embedded into the Standard Model, and the $\nu$MSM. Our study shows that sterile neutrinos can give rise to contributions to BR($Z \to \ell_1^\mp \ell_2^\pm$) within reach of the FCC-ee. We also discuss the complementarity between a high-luminosity $Z$-factory and low-energy charged lepton flavour violation facilities.