Probing the seesaw mechanism with neutrino data and leptogenesis

In the framework of the seesaw mechanism with three heavy right-handed Majorana neutrinos and no Higgs triplets we carry out a systematic study of the structure of the right-handed neutrino sector. Using the current low-energy neutrino data as an input and assuming hierarchical Dirac-type neutrino masses mDi, we calculate the masses Mi and the mixing of the heavy neutrinos. We confront the inferred properties of these neutrinos with the constraints coming from the requirement of a successful baryogenesis via leptogenesis. In the generic case the masses of the right-handed neutrinos are highly hierarchical: Mi ∝ mDi; the lightest mass is M1 ≈ 103 − 106 GeV and the generated baryon-to-photon ratio ηB . 10 −14 is much smaller than the observed value. We find the special cases which correspond to the level crossing points, with maximal mixing between two quasi-degenerate right-handed neutrinos. The two level crossing conditions are obtained: mee ≈ 0 (1-2 crossing) and d12 ≈ 0 (23 crossing), wheremee and d12 are respectively the 11-entry and the 12-subdeterminant of the light neutrino mass matrix in the basis where neutrino Yukawa couplings are diagonal. We show that sufficient lepton asymmetry can be produced only in the 1-2 crossing where M1 ≈ M2 ≈ 108 GeV, M3 ≈ 1014 GeV and (M2 −M1)/M2 . 10−5. On leave from the National Research Centre Kurchatov Institute, Moscow, Russia. E-mail address: [email protected] E-mail address: [email protected] E-mail address: [email protected]