$\nu$ Electroweak Baryogenesis: The Scalar Singlet Strikes Back

Abstract

We perform a thorough scan of the parameter space of a general singlet scalar extension of the Standard Model to identify the regions which can lead to a strong first-order phase transition, as required by the electroweak baryogenesis mechanism. We find that taking into account bubble nucleation is a fundamental constraint on the parameter space and present a conservative and fast estimate for it to enable efficient parameter space scanning. The allowed regions turn out to be already significantly probed by constraints on the scalar mixing from Higgs signal strength measurements. We also consider the addition of new neutrino singlet fields with Yukawa couplings to both scalars and forming heavy (pseudo)-Dirac pairs, as in the linear or inverse see-saw mechanisms for neutrino mass generation. Interestingly, we identify an interplay between the strength of the phase transition and the stability of the electroweak vacuum which prevents these Yukawa couplings to become arbitrarily large. Thus, their inclusion does not alter the early universe phenomenology or allowed parameter space in a significant way. Conversely, we find allowed regions of the parameter space where the presence of the neutrino singlets would remarkably modify the collider phenomenology, yielding interesting new signatures in Higgs and singlet scalar decays.