Stueckelberg Axions and Anomalous Abelian Extensions of the Standard Model

TL;DR

This thesis explores models extending the Standard Model with anomalous U(1) symmetries involving Stuckelberg axions, analyzing their theoretical structure, phenomenological implications at the LHC, and the mechanisms for anomaly cancellation.

Contribution

It provides a detailed analysis of the field theory aspects of anomalous U(1) extensions, comparing Green-Schwarz and Wess-Zumino mechanisms, and studies their collider signatures and unitarity bounds.

Findings

Anomalous gauge interactions produce small amplitudes at the LHC.

The axi-Higgs behaves as a generalized Peccei-Quinn axion with independent mass and couplings.

Unitarity bounds are identified due to anomaly poles in scattering amplitudes.

Abstract

This thesis work analyzes basic field theoretical aspects of a class of models motivated by orientifold vacua of string theory and some of their phenomenological applications at the Large Hadron Collider. They extend the gauge structure of the Standard Model by anomalous extra U(1) symmetries, which involve Stuckelberg axions for anomaly cancellation and are accompanied by Chern-Simons interactions. In particular, these effective actions are characterized by a physical pseudoscalar (the axi-Higgs) in the CP-odd spectrum, which has the properties of a generalized Peccei-Quinn axion, with independent mass and couplings to the gauge fields. Amplitudes mediated by anomalous gauge interactions are studied at the new collider in some specific channels such as Drell-Yan and double prompt-photon and shown to be small. Finally, we address the problem of the relation between the Green-Schwarz and the Wess-Zumino mechanism(s) for anomaly cancellations in effective lagrangeans involving anomalous gauge interactions, with their gauge symmetry restored either with local or non-local counterterms and identify some unitarity bounds in the Wess-Zumino case. These are due to the presence of anomaly poles in scattering amplitudes mediated by the anomaly (BIM amplitudes) and whose cancellation requires necessarly non-local counterterms. While the axi-Higgs appears in the Wess-Zumino case, two axion-like particles are needed for the cancellation of the gauge anomalies at all scales, one of them being a ghost.