Leptogenesis in Realistic Flipped SU(5)

We study thermal leptogenesis in realistic supersymmetric flipped SU (5) U (1) unification. As up-type quarks and neutrinos are arranged in the same multiplets, they exhibit strong correlations, and it is commonly believed that the masses of right-handed (RH) neutrinos are too hierarchical to fit the low-energy neutrino data. This pattern generally predicts a lightest RH neutrino too light to yield successful leptogenesis, with any lepton-antilepton asymmetry generated from heavier neutrinos being washed out unless special flavour structures are assumed. We propose a different scenario in which the lightest two RH neutrinos N₁ and N₂ have nearby masses of order 10⁹ GeV, with thermal leptogenesis arising non-resonantly from both N₁ and N₂. We show that this pattern is consistent with all data on fermion masses and mixing and predicts the lightest physical left-handed neutrino mass to be smaller than about 10^-7~eV. The Dirac phase, which does not take the maximal CP-violating value, plays an important role in leptogenesis.