Illuminating the Dark Side of the Higgs: Yukawa Couplings, Exotic Decays and Baryogenesis

This doctoral thesis covers several aspects of Higgs phenomenology in connection with flavor, dark matter, Charge-Parity (CP) violation and baryogenesis, putting emphasis on their testability at present experiments, and particularly, on their exploration at the Large Hadron Collider (LHC). We first focus on the connection between the Higgs and the flavor structure of the SM, by considering Higgs production in association with a photon, a rare process not yet observed at the LHC. We show that it is sensitive to significant deviations of Higgs couplings to first and second generation SM quarks (particularly the up-type) from their SM values, and use a multivariate neural network analysis to derive the prospects of the High-Luminosity LHC to probe deviations in the up and charm Higgs Yukawa couplings through h + γ production. We then compare the sensitivity of this channel to existing experimental searches and other methods proposed in the literature. Secondly, we study a possible connection between the Higgs and the dark matter through the exotic Higgs decays h -> Z X, with X an invisible beyond the SM particle, resulting in a semi-dark final state. Such exotic Higgs decays may occur in theories of axion-like-particles (ALPs), dark photons or pseudoscalar mediators between the SM and dark matter. The SM process h -> Z v v represents an irreducible ``neutrino floor'' background to these new physics searches, but also provides a target experimental sensitivity for them. We analyze h -> Z + invisible searches at the LHC and a future ILC, showing that these exotic Higgs decays can yield sensitivity to unexplored regions of parameter space for ALPs and dark matter models. Lastly, we explore the possibility that the CP violation (CPV) required for baryogenesis is active in the early Universe but is now suppressed. A scenario well motivated by the strong constraints placed by electric dipole moments on the existence of beyond the SM sources of CPV that could catalyze the latter. By considering CP-violating interactions between a dark and the Higgs sectors, the multi-scalar dynamics in the early Universe is able to yield a transient period of CPV enhancement, enabling a first-order EW phase transition to generate the observed baryon asymmetry. We argue that the requirement to generate a net baryon asymmetry avoiding cosmic domains naturally leads to a viable DM candidate. We study an explicit realization through a non-minimal Higgs sector consisting of two Higgs doublets and a singlet scalar odd under a Z2 symmetry, which has CP-violating interactions with the Higgs doublets. We analyze the regions of the parameter space that are consistent with perturbativity, unitarity and boundedness from below constraints where such an early Universe period of CP violation occurs, and show that the required thermal history and successful baryogenesis lead to a predictive scenario, testable by Higgs signal strengths and direct (particularly mono-jet) searches at the LHC.

Director: José Miguel No.

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