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 M-N >= 163 MeV for the normal hierarchy of neutrino masses, while M-N = 188-269 MeV and M-N >= 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 M-N = 188-350 MeV, which leads to the fact that the rate of neutrinoless double beta decay is also limited.