Type I+III Seesaw Mechanism and CP Violation for Leptogenesis

Abstract

A seesaw mechanism is presented in the neutrino sector and a new phase of CP violation ($\alpha$) emerges in the interplay between the type-I and type-III seesaw schemes. This phase is inside the mixing term, and thus it cannot be rotated away in the Yukawa Lagrangian and, therefore, the heavy symmetry states cannot be in a diagonal weak basis in the broken phase. Some particular descriptions are analyzed suggesting that if the usual Yukawa couplings are suppressed, leptogenesis still occurs due to a new interacting vertex with fermion triplet $T$, fermion singlets $N$, and an ad-hoc scalar triplet, $\Sigma$, which now is included to mediate the interactions. The evaluated CP violation is enough to generate the observed matter-antimatter asymmetry even in the minimal $1N+1T$ case (independently of $\alpha$) or in the $2N+1T$ approach (controlled by $\alpha$). The latter introduces more CP contributions to leptogenesis due to new diagrams which are now possible even with the suppressed imaginary part of the standard Yukawa couplings and can induce the observed baryon-to-photon ratio.