Unitarity bounds for gauged axionic interactions and the Green–Schwarz mechanism

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 Stückelberg 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 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.