Leptogenesis assisted by a scalar singlet

In this thesis, we study how thermal leptogenesis is modified by the addition of a complex scalar field $S$ to the type-I seesaw extension of Standard Model (SM) with $n_r$ right-handed sterile neutrinos $\nu_{R,i}$, with $i=1, \dots, n_r$. Specifically, we extend the SM by a global $U(1)_{B-L}$ symmetry under which $S$ and $\nu_{R,i}$ are charged with charges +2 and -1, respectively, while remaining singlets under the SM gauge group. After the $U(1)_{B-L}$ spontaneous symmetry breaking, with the real part of $S$ acquiring a non-vanishing vacuum expectation value, the coupling between $S$ and $\nu_{R,i}$ generates a Majorana mass term for the right-handed neutrinos and we recover the well known type-I seesaw Lagrangian. In addition, two real scalars $\phi$, massive, and $\theta$, massless, are left after the $U(1)_{B-L}$ breaking, which are coupled to the right-handed neutrinos and to the Higgs doublet. We discuss the contributions of the interactions involving $\phi$ and $\theta$ to the requisite CP-asymmetry, the production rates of right-handed neutrinos and the wash-out processes in the Boltzmann equations for leptogenesis. After identifying the leading contributions, we modify the ordinary Boltzmann equations and study the effects on the final baryon asymmetry of the Universe.