The Axion and the Goldstone Higgs

TL;DR

This paper investigates the minimal extension of a composite Higgs model to include the Peccei-Quinn symmetry, predicting specific axion-photon couplings that influence experimental searches.

Contribution

It demonstrates that only a singlet scalar needs to be added to implement the PQ symmetry in the $SO(5)/SO(4)$ model without enlarging the fermionic sector.

Findings

Minimal model requires only a singlet scalar for PQ symmetry.

Predicted axion-photon couplings are more constrained than in standard models.

Implications for axion search experiments are significant.

Abstract

We consider the renormalizable $SO(5)/SO(4)$ $\sigma$-model, in which the Higgs particle has a pseudo-Nambu-Goldstone boson character, and explore what the minimal field extension required to implement the Peccei-Quinn symmetry (PQ) is, within the partial compositeness scenario. It turns out that the minimal model does not require the enlargement of the exotic fermionic sector, but only the addition of a singlet scalar: it is sufficient that the exotic fermions involved in partial compositeness and the singlet scalar become charged under Peccei-Quinn transformations. We explore the phenomenological predictions for photonic signals in axion searches for all models discussed. Because of the constraints imposed on the exotic fermion sector by the Standard Model fermion masses, the expected range of allowed axion-photon couplings turns out to be generically narrowed with respect to that of standard invisible axion models, impacting the experimental quest.