Direct search for right-handed neutrinos and neutrinoless double beta decay

We consider an extension of the Standard Model by two right-handed neutrinos, especially with masses lighter than the charged |K| meson. This simple model can realize the seesaw mechanism for neutrino masses and also baryogenesis by flavor oscillations of right-handed neutrinos. We summarize the constraints on right-handed neutrinos from direct searches as well as big bang nucleosynthesis. It is then found that the possible range for the quasi-degenerate mass of right-handed neutrinos is |MN ≥ 163| MeV for the normal hierarchy of neutrino masses, while |MN=188|–269 MeV and |MN ≥ 285| MeV for the inverted hierarchy case. Furthermore, we find in the latter case that the possible value of the Majorana phase is restricted for |MN=188|–350 MeV, which leads to the fact that the rate of neutrinoless double beta decay is also limited.