  If any heavy neutral leptons are discovered in accelerator-based experiments,
key questions will involve their possible connection to neutrino masses or
leptogenesis. Working in a renormalisable extension of the Standard Model by
three right-handed neutrinos, we address the question of how much information
about the fundamental model parameters can be obtained by measuring the
branching ratios in the decays of the heavy neutral leptons into individual SM
generations. We find that, provided that these branching ratios could be
measured with arbitrary precision, they would be sufficient to pin down all 18
parameters of the model when supplemented with light neutrino oscillation data.
When considering a finite statistical uncertainty comparable to that which can
be achieved by future lepton colliders in the mass range of tens of GeV, some
parameter degeneracies remain, but measurements would still provide powerful
consistency checks of the model. In the sub-GeV range similar sensitivities can
be expected for SHiP and DUNE. This shows the potential of these experiments to
not only discover heavy neutral leptons, but play an important role in
understanding their role in particle physics and cosmology.
