Oscillations and leptogenesis: what can we learn about right-handed neutrinos?

Inverting the type-I seesaw formula, we reconstruct the mass matrix of the heavy right-handed neutrinos $N_i$. We analyze how the data on neutrino oscillations affect the structure of this matrix. Under the assumption of hierarchical Dirac-type neutrino masses $m_{Di}$, we compute the mixing angles among $N_i$, their masses $M_i$ and the lepton asymmetries $\epsilon_i$ generated in their decays. Unless special cancellations take place, one finds $M_i\propto m_{Di}^2$ and the generated baryon-to-photon ratio $\eta_B$ is much smaller than the observed value. We show that successful baryogenesis via leptogenesis occurs in a unique special case, which corresponds to mass degeneracy and maximal mixing of $N_1$ and $N_2$.