Searching for Triple Higgs Couplings: a Phenomenological Analysis in the Two Higgs Doublet Model

Triple Higgs couplings will be of great importance in the coming decades to probe the Higgs potential of the Standard Model (SM) of particle physics, or to unravel whether Beyond SM (BSM) physics manifests itself in the Higgs sector. In this Ph.D. thesis, we investigate the phenomenological implications of triple Higgs couplings within a two Higgs doublet model (2HDM), which extends the SM by a second Higgs doublet, which predicts the existence of two CP-even neutral Higgs bosons, h and H, one CP-odd neutral Higgs boson, A, and a pair of charged Higgs bosons, H±. We identify h with the SM-like Higgs boson discovered at the LHC. We find the allowed ranges for the tree-level triple Higgs couplings involving at least one SM-like Higgs boson by all relevant (experimental and theoretical) constraints. Our results reveal interesting ranges for the triple Higgs couplings. To explore the implications of these allowed ranges for 2HDM triple Higgs couplings, we analyze several key observables in which these scalar couplings play a crucial role. First, we study the decay of the SM-like Higgs boson into two photons. Our analysis shows that the current measurement of the diphoton signal strength can constrain some regions of the 2HDM parameter space at the 2sigma level that would otherwise be unconstrained. In particular, this can be used to also constrain the soft-breaking Z2 parameter, m12, whose dependence on the diphoton decay width only enters via the triple Higgs coupling λhH+H−. Second, we analyze the flavor-changing Higgs decay to a bottom-strange quark pair, emphasizing the role of λhH+H− in the prediction of the decay width. We find that in 2HDM type II and III the predicted values for the branching ratio can be several orders of magnitude smaller than the SM prediction. In contrast, for type I and IV we find that the predicted branching ratio can be increased up to about 70% and 50%, respectively, with respect to the SM prediction. We discuss how these deviations from the SM are caused by interference effects controlled by λhH+H−. We find that these predictions for the branching ratio are far from the expected experimental sensitivity in the medium/long term. Finally, we evaluate the double Higgs production cross sections at future high-energy e+e- colliders, such as ILC and CLIC. We consider the two main production channels of a pair of neutral Higgs bosons: the di-Higgs production together with a Z boson, and together with a neutrino-antineutrino pair. We show that the various triple Higgs-boson couplings contribute substantially in some of the studied production channels. We find regions with a strong enhancement with respect to the SM prediction of the production channel of two SM-like light Higgs bosons. We also find very large production cross sections involving one light and one heavy or two heavy 2HDM Higgs bosons, offering interesting prospects for the ILC or CLIC. The mechanisms leading to these enhanced production cross sections are analyzed in detail in this thesis. We study the cross section distributions with the invariant mass of the two final Higgs bosons, where the contributions of intermediate (resonant and non-resonant) BSM Higgs bosons containing triple Higgs couplings play a crucial role. For each proposed e+e− collider energy, we outline which process would be best suited to probe the corresponding triple Higgs-boson couplings, and analyze the interesting phenomenology involved at future colliders.