Extensions of the scalar sector of the Standard Model

Tese de mestrado, Física (Física Nuclear e Partículas) Universidade de Lisboa, Faculdade de Ciências, 2020The matter-antimatter asymmetry observed in the Universe cannot be explained by the amount of violation of Charge Conjugation and Parity (CP) in the Standard Model (SM). This shortcoming of the SM is one of the main motivations that led to the proposal of several extensions of the SM with new sources of CP-violation, a necessary requirement to explain the baryon asymmetry, according to the Sakharov conditions for baryogenesis. In this thesis, we will look at some of the main features of a particular type of models where the SM Higgs potential is changed by the addition of a second scalar doublet to the SM field content. These models are known as Two-Higgs-Doublet Models (2HDMs). They provide a very rich phenomenology at the Large Hadron Collider (LHC) and future colliders because of the introduction of new scalar states, and may answer many unsolved problems that are not addressed in the SM, such as the insufficiency of CP-violation and the existence of Dark Matter (DM). The CP-nature of the discovered Higgs is still an open issue. While it has been established by the ATLAS (A Toroidal LHC Apparatus) and CMS (Compact Muon Solenoid) collaborations that the discovered scalar cannot be a pure pseudoscalar, a mixed state with a large CP-odd component is still possible. This would be an indication of CP-violation in the scalar sector. The search for new sources of CP-violation and Beyond the Standard Model (BSM) physics is one of the main goals of the LHC. This can be achieved by a precise measurement of the Yukawa couplings since the CP-nature of the Higgs can be directly probed in its production alongside fermions. In this dissertation, we explore the sensitivity of CP-discrimination in the Higgs (h) couplings to bottom (b) and top quarks (t), for b ¯bh, bh and in dilepton final states of tth¯ events (with h → b ¯b) produced at the LHC and generated with MadGraph5 aMC@NLO. These Higgs bosons are generic, i.e., they may not correspond to the discovered one with a mass of 125 GeV, and are either pure CP-even or pure CP-odd. Several observables introduced in previous works are evaluated for a varying scalar boson mass, mh, in order to probe the CP-sensitivity in the different processes. We show that for b ¯bh and bh final states, CP-discrimination is not possible for the observables considered, even for very light Higgs masses of 10 GeV. For tth¯ , we found that distinguishing different CP states becomes increasingly difficult for larger masses, and seemingly impossible for masses above 450 GeV, at parton level. For the tops, we additionally apply an algorithm to reconstruct, for the first time, tth¯ events with a Higgs mass different from 125 GeV. Confidence Levels (CLs) for exclusion are computed for this process, as a function of the LHC luminosity, for different scenarios. We found that exclusion scenarios at the LHC require more luminosity for a fixed CL as we increase the scalar boson mass. CP-odd exclusion also requires more luminosity, relative to CP-even exclusion, for mh < 160 GeV. With the current LHC luminosity of 150 fb−1 , exclusion of a pure CP-even Higgs with a mass below 80 GeV, assuming SM-like couplings, is already possible. Also, the information that we may learn in these exclusion scenarios still leaves a large allowed parameter space for the Complex Two-Higgs-Doublet Model (C2HDM)