The Minimal Axion Minimal Linear $\sigma$ Model

Abstract

The minimal $SO(5)/SO(4)$ linear $\sigma$ model is extended including an additional complex scalar field, singlet under the global $SO(5)$ and the Standard Model gauge symmetries. The presence of this scalar field creates the conditions to generate an axion à la KSVZ, providing a solution to the strong CP problem, or an axion-like-particle. Different choices for the PQ charges are possible and lead to physically distinct Lagrangians. The internal consistency of each model necessarily requires the study of the scalar potential describing the $SO(5)\to SO(4)$, electroweak and PQ symmetry breaking. A single minimal scenario is identified and the associated scalar potential is minimised including counterterms needed to ensure one-loop renormalizability. In the allowed parameter space, phenomenological features of the scalar degrees of freedom, of the exotic fermions and of the axion are illustrated. Two distinct possibilities for the axion arise: either it is a QCD axion with an associated scale larger than $\sim 10^5$ TeV and therefore falling in the category of the invisible axions; or it is a more massive axion-like-particle, such as a $1$ GeV axion with an associated scale of $\sim 200$ TeV, that may show up in collider searches.

40 pages, 3 figures. V2 matches the version published in EPJC. V3: typo corrected and references added

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• Probing HNL-ALP couplings at colliders
• Predictive Leptogenesis from Minimal Lepton Flavour Violation
• The Axion and the Goldstone Higgs
• ALPs and HNLs at LHC and Muon Colliders: Uncovering New Couplings and Signals
• Visible GeV ALP from TeV Vector-like Leptons
• Probing Effective Field Theory Approach in the CP Violating Minimal Linear $\sigma$ Model