On the Structure of Anomalous Composite Higgs Models

TL;DR

This paper explores the anomaly structure of an extended composite Higgs model with an additional U(1) symmetry, identifying a novel non-invariant term that could lead to rare particle decays and affects phenomenology.

Contribution

It introduces a new anomaly-related term in the effective Lagrangian of a composite Higgs model with an extra U(1), expanding understanding of its phenomenological implications.

Findings

Identifies a non-invariant term similar to Wess-Zumino-Witten but not arising from anomalies.

Predicts a rare decay mode: η → h W^+ W^- Z.

Shows the model's tuning requirements are comparable to minimal models.

Abstract

We describe the anomaly structure of an composite Higgs model in which the $SO(5)/SO(4)$ coset structure of the minimal model is extended by an additional, non-linearly-realized $U(1)_{\eta}$. In addition, we show that the effective lagrangian admits a term that, like the Wess-Zumino-Witten term in the chiral lagrangian for QCD, is not invariant under the non-linearly realized symmetries, but rather changes by a total derivative. This term is unlike the Wess-Zumino-Witten term in that it does not arise from anomalies. If present, it may give rise to the rare decay $\eta \rightarrow h W^+ W^- Z$. The phenomenology of the singlet in this model differs from that in a model based on $SO(6)/SO(5)$, in that couplings to both gluons and photons, arising via anomalies, are present. We show that while some tuning is needed to accommodate flavour and electroweak precision constraints, the model is no worse than the minimal model in this regard.