Abstract :
[en] In pursuit of the origin of the masses of particles in the Standard Model, the Brout-
Englert-Higgs Mechanism predicts the existence of a scalar boson, whose discovery has
been the main goal of the Large Hadron Collider. However, introducing this scalar boson
does not provide the answer to all inadequacies of the Standard Model. Many different
Beyond Standard Model theories have been proposed in order to explain these anomalies.
Multi-Higgs-doublet models are among these models.
We restrict ourselves to the scalar sector of multi-Higgs-doublet-models. The huge
number of free parameters in the scalar potential in these models, makes it impossible to
study the most general case for any N. In order to reduce the number of free parameters,
one could impose symmetries on the scalar potential. Therefore, it is important to
explore which symmetries can be implemented in the scalar sector, how these symmetries
are broken, how they could be encoded in the Yukawa sector and what the resulting
properties of the fermions are.
Classifying these symmetries in the scalar sector is the main focus of this thesis. We
have found certain symmetries that are always broken in models with more than two
doublets, which we name ”frustrated symmetries”. Examples of such symmetries are
presented in 3HDM, and one particular symmetry, the octahedral symmetry, is studied
further. This symmetry seems to be the largest realizable discrete symmetry that can
be imposed on the scalar sector in 3HDM, and interestingly results in a 2HDM-like mass
spectrum.
In the attempt towards the classification of possible symmetries in the scalar sector
of the NHDM, we find that these symmetry groups are either subgroups of the maximal
torus, or certain finite Abelian groups which are not subgroups of maximal tori. For the
subgroups of the maximal torus, we present an algorithmic strategy that gives the full
list of possible realizable Abelian symmetries for any given N . We extend this strategy
to include Abelian antiunitary symmetries (with generalized CP transformations) in
NHDM.
We also show that multi-Higgs-doublet models can naturally accommodate scalar
dark matter candidates protected by the group Zp , since these groups are realizable in
NHDM. These models do not require any significant fine-tuning and can lead to a variety
of forms of microscopic dynamics among the dark matter candidates.
