Unitarity Bounds for Gauged Axionic Interactions and the Green-Schwarz Mechanism

TL;DR

This paper investigates unitarity bounds in anomalous gauge theories with Green-Schwarz and Wess-Zumino mechanisms, analyzing their implications for collider physics and model consistency.

Contribution

It explicitly determines unitarity bounds in models with gauged axions, comparing Green-Schwarz and Wess-Zumino mechanisms, and explores their phenomenological relevance.

Findings

Unitarity bounds depend on St"uckelberg mass and anomalous couplings.

Standard-Model-like regions are distinguishable from anomaly-dominated regions.

Bounds can reach 5-10 TeV for a Z' mass around 1 TeV.

Abstract

We analyze the effective actions of anomalous models in which a four-dimensional version of the Green-Schwarz mechanism is invoked for the cancellation of the anomalies, and we compare it with those models in which gauge invariance is restored by the presence of a Wess-Zumino term. Some issues concerning an apparent violation of unitarity of the mechanism, which requires Dolgov-Zakharov poles, are carefully examined, using a class of amplitudes studied in the past by Bouchiat-Iliopoulos-Meyer (BIM), and elaborating on previous studies. In the Wess-Zumino case we determine explicitly the unitarity bound using a realistic model of intersecting branes (the Madrid model) by studying the corresponding BIM amplitudes. This is shown to depend significantly on the St\"uckelberg mass and on the coupling of the extra anomalous gauge bosons and allows one to identify Standard-Model-like regions (which are anomaly-free) from regions where the growth of certain amplitudes is dominated by the anomaly, separated by an inflection point which could be studied at the LHC. The bound can even be around 5-10 TeV's for a $Z'$ mass around 1 TeV and varies sensitively with the anomalous coupling. The results for the WZ case are quite general and apply to all the models in which an axion-like interaction is introduced as a generalization of the Peccei-Quinn mechanism, with a gauged axion.