Utility of a Special Second Scalar Doublet

TL;DR

This paper reviews the recent interest in adding a second scalar doublet to the Standard Model, exploring the implications of a conserved symmetry that keeps its neutral component without a vacuum expectation value, affecting dark matter and neutrino mass.

Contribution

It introduces the concept of a second scalar doublet with an exact symmetry preventing a nonzero vacuum expectation value, highlighting novel phenomenological consequences.

Findings

Potential dark matter candidate from the second doublet

Implications for radiative neutrino mass generation

Connections to leptogenesis and grand unification

Abstract

This Brief Review deals with the recent resurgence of interest in adding a second scalar doublet (eta^+,eta^0) to the Standard Model of particle interactions. In most studies, it is taken for granted that eta^0 should have a nonzero vacuum expectation value, even if it may be very small. What if there is an exactly conserved symmetry which ensures <eta^0>=0? The phenomenological ramifications of this idea include dark matter, radiative neutrino mass, leptogenesis, and grand unification.