Direct Search for Right-handed Neutrinos and Neutrinoless Double Beta Decay

Abstract

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

Comments: 12 pages, 5 figures