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We computed the kinematics of Z-boson decay into a heavy-light neutrino pairwhen the Z-boson is produced at rest in electron-positron collisions, includingthe subsequent decay of the heavy neutrino into a visible final statecontaining a charged-lepton. We concentrated on heavy-neutrino masses of orderdozens of GeV and the issue of addressing the nature of the neutrinos - Diracfermions or Majorana fermions. We find that while it is not possible to tellthe nature of the heavy and light neutrinos on an event-by-event basis, thenature of the neutrinos can nonetheless be inferred given a large-enough sampleof heavy-light neutrino pairs. We identify two observables sensitive to thenature of neutrinos. One is the forward-backward asymmetry of thedaughter-charged-leptons. This asymmetry is exactly zero if the neutrinos areMajorana fermions and is non-zero (and opposite) for positively- andnegatively-charged daughter-leptons if the neutrinos are Dirac fermions. Theother observable is the polarization of the heavy neutrino, imprinted in thelaboratory-frame energy distribution of the daughter-charged-leptons. Diracneutrinos and antineutrinos produced in electron-positron collisions at theZ-pole are strongly polarized while Majorana neutrinos are at most as polarizedas the $Z$-bosons.

Z-Boson Decays into Majorana or Dirac (Heavy) Neutrinos