The axion and the Goldstone Higgs

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 is, within the partial compositeness scenario. It turns out that the minimal model does not require the enlargement of the exotic fermionic sector. The addition of a singlet scalar suffices. We explore the implementation of the symmetry and the phenomenological predictions for photonic signals in axion searches for all models discussed. Because of the constraints imposed by partial compositeness on the exotic fermion sector in order to obtain 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.