Electroweak Triplet Scalar Contribution to $SO(10)$ Leptogenesis

Abstract: We show that electroweak triplet scalar can significantly impact baryogenesis via leptogenesis in concrete and predictive $SO(10)$ GUTs, even when light neutrino masses arise predominantly from the type-I seesaw mechanism. This is illustrated within a minimal renormalisable $SO(10)$ model with $\mathbf{10}$ and $\overline{\mathbf{126}}$ scalars in the Yukawa sector and a global Peccei-Quinn-like symmetry. The quark-lepton unification and the flavour structure of the fundamental Yukawa couplings enforce type-I dominance in the light neutrino masses, also suppressing the triplet-induced CP asymmetries in the right-handed neutrino decays. However, the triplet's own decays introduce a new CP-violating source, which can enhance or suppress the total baryon asymmetry. For triplet mass near the right-handed neutrino mass scale, this contribution can dominate, making it essential in assessing the viability of $SO(10)$ leptogenesis scenarios.