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Abstract

This thesis investigates minimal extensions to the standard model (SM) scalar sector. These are separated into two parts: renormalizable and non-renormalizable. The second one is strongly motivated by dark matter while both of them get motivated by the fermion mass hierarchies. In particular, the smallness of the first fermion family masses, the dominance of the top-quark mass, or the dominance of the third fermion family masses. Additionally, the discovery of a fundamentally looking scalar particle, in agreement with the SM particle spectrum, serves as a strong reason to consider a multiscalar scenario. In the first part, we extend the SM with a second scalar doublet and consider it with the same quantum numbers as the SM Higgs. Two new models are proposed and called Type-A and B, where either the top quark alone or all third-generation fermions couple to the doublet with a larger vacuum-expectation-value (vev). This distinction becomes possible after implementing a parity symmetry and introducing the singular alignment ansatz. As a consequence, the remaining fermions exclusively acquire their masses through the small vev of the other doublet. Simultaneously, we avoid undesirable flavor-changing-neutral-currents at tree-level. We study the main differences between the proposed new models and conventional ones and include a discussion of their structure and phenomenological consequences. In the second part of this thesis, we extend the SM with a scalar singlet and a dark matter (DM) fermion. We embed this into a hybrid framework in the form of an effective completion of simplified models called extended dark matter effective field theory (eDMEFT). The phenomenology of the dimension five operators connecting the SM fermions with the dark sector is explored in the form of missing energy at several colliders in a restricted case scenario. Here we address the smallness of first-generation fermion masses via suppressed Z_2 breaking effects. The theoretical matching of the eDMEFT is performed with more-UV-complete theories such as two Higgs doublets plus a (pseudo-)scalar mediator and the inclusion of new vector-like quarks. In addition, we explore their collider signatures. Finally, we use the same framework to scrutinize the XENON1T electron recoil excess. We confront it with various astrophysical and laboratory constraints both in a general setup and in the one presented in the mentioned case scenario. We find that the excess can be explained by modified neutrino–electron interactions, linked with the neutrino and electron masses, while DM–electron scattering does not lead to statistically significant improvement. We analyze the parameter space preferred by the anomaly and find severe constraints that can only be avoided in certain corners of the parameter space. In particular, problematic bounds on electron couplings from Big-Bang Nucleosynthesis can be circumvented via a late phase transition in the new scalar sector.