Scalar and fermionic extensions of the Standard Model

In this thesis several aspects of beyond SM physics schemes have been treated. In particular, two categories of models have been considered: ($i$) models with extra
Higgs ElectroWeak (EW) doublets (multi-Higgs-doublet models), ($ii$) models with new fermion EW singlets (type-I seesawmodels). In the first category, two problems
associated with the most general two-Higgs-Doublet Model (2HDM) and
with the three-Higgs-Doublet Model (3HDM) have been tackled. In the
former case the scalar mass spectrum has been derived in a
basis-invariant fashion whereas in the later, after introducing a
general procedure for the minimization of highly symmetric potentials,
the minimization of an $S_4$ and of an $A_4$ 3HDM has been
analyzed. In the second category, the possibility of envisaging
seesaw-like models yielding sizeable lepton-flavor-violating decay
rates has been investigated. With the models at hand, the
corresponding charged lepton-flavor-violating phenomenology has been
studied focusing on rare muon decays, for which forthcoming
lepton-flavor-violating experiments will be able to prove large parts
of their parameter space.