Nonstandard Types of Mixing and Dark Matter in Models with Heavy Neutral Leptons

The extension of the Standard Model by three right-handed heavy leptons in the physical basis of mass states is considered. The cosmological restrictions and constraints of direct searches for each of the three heavy neutral leptons are analyzed together within the framework of a unified parameterization of the mixing components with active neutrinos. The possible contribution of the nonzero mass of the lightest neutrino is taken into account. Within the framework of this mixing model, the applicability of phenomenological simplifications for mixing components is analyzed.