Hybrid Resonant Type-I and Type-II Leptogenesis in SO(10) with Quasi-Degenerate Triplet and Right-Handed Neutrino Masses $M_T \simeq M_{N_3}$

Abstract

The origin of the baryon asymmetry of the Universe remains one of the central open problems in particle physics and cosmology. We identify a regime of \emph{hybrid resonant leptogenesis} in renormalizable SO(10) grand unified theories, where a quasi-degeneracy between the scalar triplet and the heaviest right-handed neutrino, \( M_T \simeq M_{N_3}, \) naturally arises from their common origin in unified scalar multiplets and leads to an enhanced interference between type-I and type-II decay amplitudes. This interference gives rise to a physical CP-violating phase, \( \phi_{\rm HR} = \arg(\mu f Y_\nu^\dagger), \) which cannot be removed by field redefinitions and induces a resonant enhancement of the baryon asymmetry without requiring extreme Yukawa hierarchies or fine-tuned mass splittings. We show that the observed asymmetry, \( Y_B \simeq 8.7 \times 10^{-11}, \) can be reproduced for mass scales around \(10^{11}\,\mathrm{GeV}\), moderate washout effects, and $\mathcal{O}(1)$ CP-violating phases. The mechanism leads to correlated low-energy signatures in lepton-flavour violation and electric dipole moments, providing potential experimental tests.