In this thesis we consider extending the standard model of particle physics (SM) to include a fourth generation of elementary particles (SM4). The fourth generation would have to be sufficiently heavy to have escaped detection; specifically, its neutrino is required to be kinematically inaccessible to the Z boson in order to agree with the very precise LEP measurements of the Z width. This extension is appealing since the current theory (the SM) exhibits tension with some phenomena observed in nature. Such phenomena are, for example, the replication and number of fermion families, the ratio of matter to antimatter in the observable universe, charge-parity violation and the mixings between the fermions.
Up to very recently the issue of the origin of mass, that is, the mechanism of electroweak symmetry breaking in the SM, was lacking experimental verification. However, during the writing of this thesis there have been some very exciting advances in this domain. In July 2012 the CMS and ATLAS collaborations at the LHC have reported the observation of a resonance at ∼ 125 GeV, an observation confirmed by experiments at another high-energy particle collider, the Tevatron. This resonance seems to correspond to the Higgs particle, the quantum of the scalar field responsible for the breaking of the electroweak symmetry.
Besides verifying the answer to the theoretically fundamental question about the origin of mass, the experimental discovery also serves as a constraint for any theory of particle physics. The goodness of models describing particle physics are generally tested by performing global fits to the data, with the data set usually taken to be the most precisely measured quantities available — the electroweak precision observables. When the recent Higgs signal strengths are included in the data set, it is seen that the SM4 is not a correct theory of nature. Specifically, the Higgs signals predicted by the SM4 are not in agreement with the data, and the model has in September 2012 been quite decisively excluded at the statistical significance of 5.3σ.
Following the developments in the field we next consider the phenomenological effects of adding another scalar doublet to the previously considered SM4. In the SM and SM4 there is just one scalar (Higgs) doublet: the models have a minimal scalar sector. The fermion sector, however, is not minimal: there are at least three replicas of a fermion family and so it is possible that the scalar sector is not minimal either. There are in fact arguments in favor of several Higgs doublets, for example supersymmetry and the baryon asymmetry of the universe. Two doublets give rise to five physical particles and so the phenomenology of such models is much richer than in the minimal scenario.
Four family-models have received a great deal of interest in the last decade: some 500 articles are reported to have been published concerning their phenomenology during this time. This thesis is a review of the recent developments is this field
