Revealing neutrino mass ordering at the CEPC and FCC-ee

The ordering of neutrino masses remains a key unknown in particle physics and cosmology. While upcoming oscillation experiments are expected to determine the mass ordering at low energies, it is important to explore complementary probes that access the underlying mechanism of neutrino mass generation. In this work, we show that future high-energy electron-positron colliders can provide sensitivity to the neutrino mass ordering through the lepton-flavor structure of heavy neutral lepton (HNL) interactions. In the minimal type-I seesaw scenario with two nearly degenerate HNLs, the flavor composition of the heavy–light neutrino mixings is strongly correlated with the light-neutrino mass spectrum, leading to distinct collider signatures for normal and inverted mass orderings. We demonstrate that future Z factories such as CEPC and FCC-ee can probe these flavor patterns over a wide region of parameter space, establishing collider searches for HNLs as a complementary approach to neutrino mass ordering studies.