A global approach to physics beyond the Standard Model

Abstract

The discovery of the Higgs boson marked an extraordinary moment in particle physics as the last piece of the Standard Model was experimentally verified. However, the quest for a fundamental theory of Nature continues as the Standard Model still faces several challenges, both from experimental evidence and from a theoretical perspective, prompting the proposal of a tremendous amount of beyond the Standard Model scenarios. This thesis follows two complementarity directions in the search for new physics: the first one, the model-driven approach, requires the selection of a model or class of models and explores their most unique signatures; the second one, valid if new physics is heavy, is to follow the effective field theory approach and parametrize new physics effects in a model-independent way. While the model-driven approach encompasses, by definition, less beyond the Standard Model scenarios, we can develop strategies so that the setup is applicable to a wide range of models; furthermore, if new physics predicts light degrees of freedom, then this approach must be followed. We proceed with this strategy focusing on composite Higgs models, motivated theories which explain the hierarchy problem by considering the Higgs as a pseudo-Nambu Goldstone boson. A ubiquitous prediction of the composite framework is the existence of vector-like leptons. As such, we propose dedicated analysis to search for these particles at colliders considering a generic setup for their decays and study the possible complementarity with dark matter probes. In the model-independent approach, we extend the current calculations within the Standard Model Effective Field Theory to account for the one-loop contribution of dimension-8 terms. These terms are important not only from the experimental point of view, since certain observables receive their main contribution at this order, but also because dimension-8 Wilson coefficients are the first ones on which purely theoretical bounds can be placed. Furthermore, we consider the extension of the Standard Model Effective Field Theory with an extra light pseudo-scalar, a motivated scenario within the composite framework. Finally, we use this model-independent approach to connect experimental results with Standard Model extensions by classifying models which can generate the effective operators possibly responsible for the anomalous magnetic moment of the muon.