Phenomenological aspects of elementary and composite Higgs models

Abstract

This thesis presents the phenomenological analysis of the charged Higgs sector in 3-Higgs Doublet Models (3HDMs) and that of heavy Higgs bosons in Composite 2-Higgs Doublet Models (C2HDMs). In the first chapter, the main properties of five types of 3HDMs are summarised. In addition, the constraints from the measurements of the B > X_sγ decay rate on the parameter space of 3HDMs are considered at the next-to-leading order in QCD and then limits from direct searches for H₁^± and H₂^± at colliders are discussed. In the other chapters, we move on to the investigation of a C2HDM framework based on the spontaneous breakdown of a global symmetry SO(6) → SO(4) × SO(2) at the compositeness scale f. The motivations and the main properties of it are summarised. By requiring C2HDMs to full perturbative unitarity at energies, we obtain constraints on its parameters. We also study the Large Hadron Collider (LHC) phenomenology of C2HDMs of various Yukawa types by concentrating on the modifications from Standard Model (SM) couplings of the discovered Higgs state (h) in addition to the production cross sections and Branching Ratios (BRs) of additional Higgs bosons. We find that, even if the same deviation in the hV V (V = W^±,Z) coupling is established in both a C2HDM and Elementary 2-Higgs Doublet Model (E2HDM), one can disentangle the two scenarios due to the structure of the Yukawa couplings by using the production and decay properties of additional Higgs bosons. Finally, we study single- and double-h production at future e⁺e⁻ colliders in C2HDMs and E2HDMs. In particular, we study the differences in the cross sections between the two hypotheses when the deviation in the hV V (V = W^±,Z) coupling is set to be the same value in both scenarios.

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ORCID for Stefano Moretti: orcid.org/0000-0002-8601-7246

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