Abstract
We discuss the implications of a recently proposed minimally extended SU(5) model that incorporates the superlight axion mechanism to solve the strong CP problem. We prove that in the absence of instanton effects, the pseudoscalar nature of the axion coupling to fermionic matter is not modified by higher order weak radiative corrections including CP violation, thus leading to the induced scalar coupling of axion to matter ∼ƒ π/M U ·θ . We also note that the axion coupling to light Hi bosons is suppressed in a similar manner by the heavy mass, M U in the absence of instanton effects, thus making the GUT axion practically invisible. Finally, we note that in this model the baryon asymmetry of the Universe cannot be generated in lowest order - a problem which can be solved by the introduction of extra Higgs mesons.
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