Phenomenology in multi-scalar extensions of the Standard Model

Sammanfattning: This thesis is composed of four papers, which all treat various extensions to the Standard Model (SM). The first two papers concern a particular supersymmetric, grand-unified theory (GUT), while in the latter two, we classify anomaly-free implementations of two-Higgs doublet models (2HDM) with a gauged abelian symmetry. Paper I. In this paper, we present an alternative solution for avoiding GUT-scale lepton masses in so-called trinification based GUT-models (T-GUTs). Rather than introducing several copies of the tri-triplets, which is the conventional approach, we solve the issue by embedding the symmetries into an E8 group.Paper II. This paper involves a more detailed study of the model proposed in Paper I. Some of its features is an absence of the mu-problem and a universal Yukawa coupling for all chiral fermions. Paper III. Here, we classify all anomaly-free implementations of a 2HDM with a gauged Abelian symmetry and the SM fermion content. The resulting 11 models are then compared under a range of experimental bounds, from which we identify the most promising ones. Paper IV. In this paper, we again classify and compare all anomaly-free implementations of a 2HDM with a gauged Abelian symmetry, but this time around also including two generations of right-handed neutrinos. The right-handed neutrinos are incorporated to provide a mechanism for the neutrinos to obtain a mass, here via a type-I seesaw mechanism. In total, there are 16 valid models, which are all compared, both for normal- and inverted ordering. Two models are identified as the most promising candidates for future studies.