Phenomenology of Models with New Scalar Particles

The discovery of a 125 GeV Higgs boson is one of the greatest successes of the Standard Model of particle physics, but it is also one of the paths to finding its failings. The scalar sector of the Standard Model is only a minimal implementation of electroweak symmetry breaking. In spite of a rich sector of fermions and gauge bosons, the Standard Model predicts the existence of only one scalar particle, and the properties of this scalar particle are fully determined. Precisely measuring the properties of this Higgs boson is thus one of the main paths avenues to looking for new physics. A detailed analysis of the Higgs boson inevitably leads to studying the phenomenology of extended scalar sectors. The vast landscape of models with extended scalar sectors is also relevent many of the unanswered questions in particle physics. This dissertation aims to demonstrate the many possible phenomenological consequences of new scalars. We will explore some scalar models with important phenomenological consequences relevant to the frontiers of fundamental physics. We first cover benchmarks of the simplest scalar extensions. These extensions can greatly change the picture of electroweak symmetry breaking. Such models also can lead to large cross sections for the production of pairs of scalars, an important signal to search for at colliders. Then we move on to a more complicated scalar sector with three Higgs doublets. This extension can accomodate a mechanism that explains the asymmetry between matter and anti-matter in the universe while also predicting observable consequences for Higgs boson properties and production of new scalars. We then move on to dark sectors that connect the Standard Model to dark matter. The properties of dark matter are largely unknown, but scalar portals are one potential avenue of study. In one model, we examine the possibility of connecting dark matter and neutrino masses with a scalar portal. This provides a novel explanation of neutrino masses with potentially observable consequences for cosmic neutrino detection. In the final model, we discuss the combination of a dark sector, with a dark force and dark Higgs mechanism, and a popular vector like quark model. The inclusion of dark sector particles turns out to vastly change the phenomenology of this popular model, with entirely different decay patterns for the vector like quark. We conclude that Standard Model extensions containing new scalars have immense potential for new physics and answers to open questions in the field