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 $M_N \geq 163 \MeV$ for normal hierarchy of neutrino masses, while $M_N = 188 \text{--} 269 \MeV$ and $M_N \geq 285 \MeV$ for inverted hierarchy case. Furthermore, we find in the latter case that the possible value of the Majorana phase is restricted for $M_N = 188 \text{--} 350 \MeV$, which leads to the fact that the rate of neutrinoless double beta decay is also limited.
Highlights
Various oscillation experiments have revealed non-zero masses of neutrinos
When the Majorana masses are much heavier than the Dirac ones, the smallness of neutrino masses can be explained by the seesaw mechanism [2]
The successful scenario of baryogenesis can be realized even if the masses are smaller than the electroweak scale [9, 10]. In these extensions of the Standard Model, the detailed examination of right-handed neutrinos is crucial in order to elucidate the mechanism to generate neutrino masses as well as the cosmic baryon asymmetry
Summary
Various oscillation experiments have revealed non-zero masses of neutrinos. The observation shows that there exist two mass scales of neutrinos, the differences of mass squared ∆m2atm ≃ 2.43 × 10−3 eV2 and ∆m2sol ≃ 7.54 × 10−5 eV2 [1] , related to the so-called atmospheric and solar neutrinos, respectively. The successful scenario of baryogenesis can be realized even if the masses are smaller than the electroweak scale [9, 10] In these extensions of the Standard Model, the detailed examination of right-handed neutrinos is crucial in order to elucidate the mechanism to generate neutrino masses as well as the cosmic baryon asymmetry. For this purpose, the scenario with lighter νR’s is more promising. It will be discussed that this leads to the important impact on the 0ν2β experiments in future
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