  The neutrino masses and flavor mixings, which are missing in the Standard
Model (SM), can be naturally incorporated in the type-I seesaw extension of the
SM with heavy Majorana neutrinos being singlet under the SM gauge group. If the
heavy Majorana neutrinos are around the electroweak scale and their mixings
with the SM neutrinos are sizable, they can be produced at high energy
colliders, leaving characteristic signatures with lepton-number violations.
Employing the general parametrization for the neutrino Dirac mass matrix in the
minimal seesaw scenario, we perform a parameter scan and identify allowed
regions to satisfy a variety of experimental constraints from the neutrino
oscillation data, the electroweak precision measurements and the lepton-flavor
violating processes. We find that the resultant mixing parameters between the
heavy neutrinos and the SM neutrinos are more severely constrained than those
obtained from the current search for heavy Majorana neutrinos at the LHC. Such
parameter regions can be explored at the High-Luminosity LHC and a 100 TeV
pp-collider in the future.
