Perturbative and Nonperturbative Aspects of the Phenomenology of Composite Higgs Models

Abstract

The discovery of the Higgs boson announced by CERN on July 4th 2012 was expected by the particle physics community since a long time. Indirect evidence that a Higgs boson must be present in the mass range of 100–200 GeV was provided by electroweak precision data collected at LEP already ten years ago. Yet, the existence of a fundamental scalar in nature poses a strong problem regarding the behaviour of the theory at high energy. The relative smallness of the Higgs mass compared to the scale at which new physics must appear is known as the hierarchy problem. So far, only two solutions to this problem can be formulated in terms of a local field theory in 3 + 1 dimensions, namely supersymmetry and strong dynamics. The second option and the phenomenological consequences thereof are considered in this thesis. In this case, the Higgs boson arises as a composite state of new fermions confined by the strong interaction. Models with a new, strongly-coupled sector are also the natural four-dimensional effective description of theories based on extra-dimensions. In this work, the requirements for building a realistic composite Higgs model are studied. In the simplest models, the natural scale at which new physics appears is close to the electroweak scale and should therefore be testable at the LHC. Vector resonances, new fermions coupling mostly to the top quark and anomalous Higgs couplings are the main signature of these models. Alternatively, models in which the light resonances are absent or hidden and for which the Higgs couplings mimic the Standard Model ones require the presence of larger symmetries. As a side effect, these models can contain additional massive, stable particles which are the equivalent of baryons in quantum chromodynamics. These particles arise in the low-energy effective description of the model as topological solitons, called skyrmions. The skyrmion’s properties are studied first in a general framework and then in specific realisations of composite Higgs models. Despite being naturally very massive and weakly-interacting, which seems to indicate potential dark matter candidates, the stable skyrmion states are often electrically charged and incompatible with the early-universe cosmology.