Mixing of Active and Sterile Neutrinos

Original · EN

We investigate mixing of neutrinos in the νMSM (neutrino Minimal Standard Model), which is the MSM extended by three right-handed neutrinos. Especially, we study elements of the mixing matrix Θαᵢ between three left-handed neutrinos να (α= e,μ,τ) and two sterile neutrinos Nᵢ (I=2,3) which are responsible to the seesaw mechanism generating the suppressed masses of active neutrinos as well as the generation of the baryon asymmetry of the universe (BAU). It is shown that ΘeI can be suppressed by many orders of magnitude compared with Θμᵢ and Θτᵢ, when the Chooz angle θ₁₃ is large in the normal hierarchy of active neutrino masses. We then discuss the neutrinoless double beta decay in this framework by taking into account the contributions not only from active neutrinos but also from all the three sterile neutrinos. It is shown that N₂ and N₃ give substantial, destructive contributions when their masses are smaller than a few 100 MeV, and as a results Θₑ ᵢ receive no stringent constraint from the current bounds on such decay. Finally, we discuss the impacts of the obtained results on the direct searches of N₂,₃ in meson decays for the case when N₂,₃ are lighter than pion mass. We show that there exists the allowed region for N₂,₃ with such small masses in the normal hierarchy case even if the current bound on the lifetimes of N₂,₃ from the big bang nucleosynthesis is imposed. It is also pointed out that the direct search by using π+ → e+ + N₂,₃ and K+ → e+ + N₂,₃ might miss such N₂,₃ since the branching ratios can be extremely small due to the cancellation in ΘeI, but the search by K+ → μ+ + N₂,₃ can cover the whole allowed region by improving the measurement of the branching ratio by a factor of 5.