Scalar-singlet assisted leptogenesis with CP violation from the vacuum

TL;DR

This paper investigates a novel leptogenesis mechanism where CP violation originates from a scalar singlet's vacuum expectation value, introducing new contributions to CP asymmetries and linking high-energy CP violation with low-energy neutrino data.

Contribution

It introduces a leptogenesis framework with CP violation from a scalar singlet, including detailed calculations of new CP asymmetries and a minimal model with a $ ext{Z}_8$ symmetry.

Findings

New scalar singlet induces additional CP asymmetries.

Model links high-energy CP violation to low-energy neutrino phases.

Standard CP contributions vanish due to symmetry, enabling alternative leptogenesis pathways.

Abstract

In the vanilla type-I seesaw leptogenesis scenario, CP violation required to generate the lepton asymmetries in the heavy Majorana neutrino decays stem from complex Dirac-type Yukawa couplings. In this paper we explore the case in which that CP violation originates from the vacuum expectation value of a complex scalar singlet at a very high scale. This non-trivial CP-violating phase can be successfully communicated to the low-energy neutrino sector via the heavy neutrino portal. The new scalar-singlet degrees of freedom generate new contributions to the CP asymmetries relevant for leptogenesis not only at the one-loop level but also through tree-level three-body decays. These are computed here for an arbitrary number of heavy neutrinos, Higgs doublets and scalar singlets. We also take into account the new decays and scattering processes that enter the unflavoured Boltzmann equations governing the heavy-neutrino particle densities and the $(B-L)$-asymmetry evolution. Having established the framework of interest, we present a simple model with two RH neutrinos, two Higgs doublets and a complex scalar singlet, supplemented with a $\mathcal{Z}_8$ flavour symmetry. This symmetry minimises the number of free parameters without compromising the possibility of spontaneous CP violation and compatibility with neutrino data. In fact, the only viable $\mathcal{Z}_8$ charge assignment shows a preference for a non-trivial spontaneous CP-violating phase, which in turn leads to a non-vanishing CP asymmetry due to the direct link between high- and low-energy CP violation. An interesting feature of this simple setup is that the usual wave and vertex type-I seesaw contributions to the CP asymmetry vanish due to the $\mathcal{Z}_8$ symmetry. Thus, the observed baryon-to-photon ratio can be explained thanks to the new couplings among the heavy neutrinos and the new scalar degrees of freedom.