Minimal Seesaw Model with a Discrete Heavy-Neutrino Exchange Symmetry
  We present a Froggatt-Nielsen flavor model that yields a minimal realization of the type-I seesaw mechanism. This seesaw model is minimal for three reasons: (i) It features only two rather than three right-handed sterile neutrinos: N_1 and N_2, which form a pair of pseudo-Dirac neutrinos; (ii) the neutrino Yukawa matrix exhibits flavor alignment, i.e., modulo small perturbations, it contains only three independent parameters; and (iii) the N_1,2 coupling to the electron flavor is parametrically suppressed compared to the muon and tau flavors, i.e., the neutrino Yukawa matrix exhibits an approximate two-zero texture. Crucial ingredients of our model are (a) Froggatt-Nielsen flavor charges consistent with the charged-lepton masses as well as (b) an approximate, discrete exchange symmetry that manifests itself as N_1 <--> i N_2 in the heavy-neutrino Yukawa interactions and as N_1 <--> N_2 in the heavy-neutrino mass terms. This model predicts a normal light-neutrino mass hierarchy, a close-to-maximal CP-violating phase in the lepton mixing matrix, \delta~3/2\pi, as well as resonant leptogenesis in accord with arguments from naturalness, vacuum stability, perturbativity and lepton flavor violation. 
Thomas Rink, Kai Schmitz, Tsutomu T. Yanagida
hep-ph