  The Standard Model of particle physics is still lacking an understanding of
the generation and nature of neutrino masses. A favorite theoretical scenario
(the see-saw mechanism) is that both Dirac and Majorana mass terms are present,
leading to the existence of heavy partners of the light neutrinos, presumably
massive and nearly sterile. These heavy neutrinos can be searched for at high
energy lepton colliders of very high luminosity, such as the Future
electron-positron e+e- Circular Collider, FCC-ee (TLEP), presently studied
within the Future Circular Collider design study at CERN, as a possible first
step. A first look at sensitivities, both from neutrino counting and from
direct search for heavy neutrino decay, are presented. The number of neutrinos
should be measurable with a precision between 0.001 - 0.0004, while the direct
search appears very promising due to the long lifetime of heavy neutrinos for
small mixing angles. A sensitivity down to a heavy-light mixing of 10^{-12} is
obtained, covering a large phase-space for heavy neutrino masses between 10 and
80 GeV/c2.

High Energy Physics - Experiment;High Energy Physics - Phenomenology