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

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 -> l(1)(-/+)l(2)(+/-) 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 wellmotivated 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 -> l(1)(-/+)l(2)(+/-)) 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.