Leptogenesis in Realistic Flipped SU(5)

TL;DR

This paper explores a new scenario for thermal leptogenesis in supersymmetric flipped SU(5), where two nearly degenerate right-handed neutrinos generate the lepton asymmetry non-resonantly, aligning with fermion data and predicting an extremely light neutrino.

Contribution

It introduces a novel leptogenesis mechanism with nearly degenerate right-handed neutrinos in flipped SU(5), compatible with observed fermion masses and mixing.

Findings

Successful non-resonant leptogenesis from two nearly degenerate RH neutrinos.

Predicted lightest neutrino mass below 10^{-7} eV.

Dirac phase influences leptogenesis significantly.

Abstract

We study thermal leptogenesis in realistic supersymmetric flipped $SU(5)\times U(1)$ 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 $N_1$ and $N_2$ have nearby masses of order $10^9$ GeV, with thermal leptogenesis arising non-resonantly from both $N_1$ and $N_2$. 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.