Higgs Couplings Analysis Via Effective Field Theories Restores Unitarity at One Loop

The search for physics beyond the Standard Model receives a significant boost from new research exploring the subtle behaviour of the Higgs boson, a fundamental particle responsible for giving mass to other particles. I. Asiáin and colleagues investigate how precise measurements of the Higgs boson’s interactions, known as Higgs couplings, can reveal evidence of previously unknown forces or particles. This work develops a sophisticated theoretical framework combining effective field theories with unitarization techniques, methods that ensure calculations remain physically realistic at high energies, and applies it to the study of Higgs dynamics. By meticulously analysing these interactions, the team demonstrates how to impose fundamental constraints on the Higgs boson’s self-interactions and, crucially, extract information about the Higgs sector from particle collisions without needing to directly observe rare double-Higgs production events. This approach offers a powerful new avenue for probing the mysteries beyond our current understanding of the universe.

de Física Quàntica i Astrofísica (FQA) de la Universitat de Barcelona. This department has genuinely felt like my home, and it has been a pleasure to develop my work as both a researcher and a teacher. This thesis would not have been possible without my supervisors, Dr. Espriu and Dr. Mescia.

I thank Domènec and Federico for all the hours of collaborative work, delivered with enormous patience and understanding, especially during times when I felt most lost. The success of this project is due not only to your undoubted professional quality, but also to the personal qualities you have demonstrated. I appreciate every piece of advice and the different perspectives offered throughout this process.

Four-Lepton Events and Weak Boson Scattering

Researchers have extensively investigated the properties of fundamental particles and forces, particularly those related to the Higgs boson and electroweak interactions. Studies focus on analysing data from particle collisions to refine our understanding of these interactions and search for new physics beyond the Standard Model. These analyses include precise measurements of four-lepton events, providing insights into particle production and decay, and exploring the behaviour of weak bosons at high energies, contributing to a more complete picture of the universe’s fundamental building blocks and governing forces. Significant work also focuses on understanding gravity, cosmology, and quantum gravity.

Researchers explore the nature of gravitational waves, developing theoretical models and conducting experiments to detect these ripples in spacetime. Investigations into extra dimensions and modified gravity theories aim to address fundamental questions about the universe’s expansion and the nature of dark energy, contributing to our understanding of the universe’s origins, evolution, and ultimate fate. Key authors consistently contributing to these fields include Delgado, Dobado, Espriu, and Oller, whose research has advanced our understanding of particle physics and gravity, forming the foundation for ongoing investigations into the fundamental laws of nature.

Unitarization Restores Validity of Effective Field Theories

This work investigates the potential of effective field theories, combined with unitarization techniques, to explore physics beyond the Standard Model, focusing on the origin of electroweak symmetry breaking. Researchers addressed the issue of unitarity violation inherent in effective theories by developing methods to restore unitarity before comparing theoretical predictions with experimental data. The study began with a detailed one-loop calculation within the Higgs Effective Field Theory (HEFT) framework, determining necessary counterterms using the on-shell scheme. This allowed for analysis of how including transverse gauge bosons modifies the masses and widths of dynamical resonances in both vector-isovector and scalar-isoscalar channels, establishing technical tools for studying low-energy couplings within the Higgs effective theory, ensuring adherence to unitarity and causality.

While vector resonances are well-understood, scalar resonances remain less constrained and depend on specific HEFT parameters. By applying unitarization techniques alongside the requirement of causality, scientists derived nontrivial bounds on Higgs self-interactions, demonstrating the power of this approach. Crucially, considering coupled channels during unitarization enabled the extraction of information about the Higgs sector from elastic scattering processes, even without requiring double-Higgs production. Extending this methodology, the researchers successfully applied their framework to unitarize amplitudes from a theory of gravity, treating it as an effective field theory, revealing the spectrum of scalar resonances that emerge in Einstein-Hilbert gravity when higher-order operators are added to the expansion, showcasing the versatility of the approach in exploring diverse theoretical landscapes. The results provide a powerful new toolkit for investigating fundamental physics and pushing the boundaries of our understanding of the universe.

Unitarity Constrains Electroweak Symmetry Breaking Resonances

This work advances the understanding of how effective field theories and unitarization techniques can explore physics beyond the Standard Model, specifically focusing on the origin of electroweak symmetry breaking. Researchers performed detailed calculations within the framework of the Higgs Effective Field Theory, carefully accounting for quantum loop effects and ensuring the mathematical consistency of the calculations at high energies. This allowed for a comprehensive analysis of how the inclusion of transverse gauge bosons influences the properties of resonances predicted by the theory in both vector and scalar channels, demonstrating that unitarization, combined with the principle of causality, places significant constraints on the self-interactions of the Higgs boson, offering a novel approach to probing the Higgs sector through scattering processes without relying on the observation of rare double-Higgs production. Importantly, the research highlights the necessity of considering coupled processes when analysing scalar resonances, providing a more accurate description of their behaviour.

The methodology developed was successfully applied to a theory of gravity, demonstrating the broad applicability of these techniques beyond the Standard Model. The authors acknowledge that the precise determination of scalar resonance properties remains challenging due to experimental difficulties in constraining relevant parameters. Future research directions include refining the analysis of specific resonances and extending the methodology to explore other beyond-the-Standard-Model scenarios. This work provides a valuable theoretical framework for interpreting experimental data and guiding future searches for new physics at high-energy colliders.