Anomalous Higgs Couplings as a Window to New Physics

TL;DR

This thesis explores how effective field theories and unitarization techniques can probe new physics beyond the Standard Model, focusing on electroweak symmetry breaking and Higgs couplings through detailed scattering amplitude analysis.

Contribution

It introduces a comprehensive framework combining one-loop HEFT calculations, unitarization, and causality constraints to analyze Higgs and gauge boson interactions, including gravity as an EFT.

Findings

Unitarization constrains Higgs self-interactions.

Scalar resonances depend on difficult-to-measure HEFT parameters.

Boundaries on Higgs couplings are derived from causality and unitarity.

Abstract

Throughout this thesis, we investigate how effective field theories, combined with unitarization techniques, can be used to explore physics beyond the Standard Model, with particular emphasis on the dynamical origin of electroweak symmetry breaking. Since effective theories often produce amplitudes that violate unitarity at high energies, restoring unitarity is essential before comparing predictions with data. The first part of the work focuses on longitudinal $WW$ scattering, a process that, despite being subdominant at the LHC, provides a sensitive probe of Higgs dynamics. We perform a full one-loop computation within the HEFT framework, determine the required counterterms in the on-shell scheme, and analyze for the first time how the inclusion of transverse gauge bosons modifies the masses and widths of the dynamical resonances obtained through unitarization in the vector--isovector and scalar--isoscalar channels. This provides the technical foundation for studying HEFT low-energy couplings subject to unitarity and causality. While vector resonances are relatively well characterized through Weinberg sum rules and unitarization, scalar resonances remain more elusive and depend on HEFT parameters that are experimentally difficult to constrain. We show that unitarization, together with causality, imposes nontrivial bounds on Higgs self-interactions. In the scalar channel, the necessity of treating coupled processes becomes a key ingredient, enabling us to extract information about the Higgs sector from elastic $WW \to WW$ scattering without requiring double-Higgs production. Finally, we apply the formalism developed throughout the thesis to unitarize the amplitudes of a quantum theory of gravity treated as an effective field theory, illustrating the broader applicability of these methods.