Beyond the minimal composite Higgs model

TLDR: In this paper, the SO(6)/SO(5) model is extended with a CP-odd singlet scalar, η, in addition to the Higgs doublet.

Abstract: The Higgs spectrum of the minimal composite Higgs model, based on the SO(5)/SO(4) coset, consists of a unique Higgs doublet whose phenomenology does not differ greatly from the Standard Model (SM). Nevertheless, extensions beyond this minimal coset structure exhibit a richer Higgs spectrum and therefore very different Higgs physics. We explore one of these extensions, the SO(6)/SO(5) model, whose Higgs spectrum contains a CP-odd singlet scalar, η, in addition to the Higgs doublet. Due to the pseudo-Nambu- Goldstone nature of these Higgs bosons, their physical properties can be derived from symmetry considerations alone. We find that the mass of η can be naturally light, opening up the possibility that the SM Higgs decays predominantly to the singlet, and therefore lowering the LEP bound on its mass to 86GeV. We also show that η can have interesting consequences in flavour-violating processes, as well as induce spontaneous CP-violation in the Higgs sector. The model can also have anomalies, giving rise to interactions between the SM gauge bosons and η which, if measured at the LHC, would give quantitative information about the structure of the high energy theory.