Introducing tools to test Higgs boson interactions via WW scattering. II. The coupled-channel formalism and scalar resonances

In this work, we explore in detail the presence of scalar resonances in the WW fusion process in the context of the LHC experiments working in the theoretical framework provided by Higgs effective field theories (HEFTs). While the phenomenology of vector resonances is reasonably understood in the framework of Weinberg sum-rules and unitarization studies, scalar resonances are a lot less constrained and, more importantly, do depend on HEFT low-energy effective couplings different from the ones of vector resonances that are difficult to constrain experimentally. More specifically, unitarization techniques combined with the requirement of causality allows us to set nontrivial bounds on Higgs self-interactions. This is due to the need for considering coupled channels in the scalar case along the unitarization process. As a byproduct, we can gain some relevant information on the Higgs sector from WW & RARR; WW elastic processes without needing to consider two-Higgs production.