Improved Cosmological Bounds for Mixing Scenarios of Three Sterile Neutrino Generations

The gauge SU(2)_L × U(1) model with the extension of the lepton sector by three right-handed Majorana sterile neutrinos is considered. The complete mass matrix of 6 × 6 active and sterile neutrinos is diagonalized. The cosmological bounds following from the lifetime of sterile neutrinos and the fraction of energy carried by sterile neutrino dark matter are obtained. The deviations from the “fine-tuned” mixing scenario sensitive to the mass of the lightest standard (active) neutrino are considered, and the regions of the model parameter space corresponding to these deviations are distinguished.