Constraining scalar-Gauss-Bonnet inflation by reheating, unitarity, and Planck data

We revisit the inflationary dynamics in detail for theories with Gauss-Bonnet gravity coupled to scalar functions, in light of the Planck data. Considering the chaotic inflationary scenario, we constrain the parameters of two models involving inflaton-Gauss-Bonnet coupling by current Planck data. For nonzero inflaton-Gauss-Bonnet coupling β, an inflationary analysis provides us a big cosmologically viable region in the space of (m, β), where m is the mass of the inflaton. However, we study further on constraining β arising from reheating considerations and unitarity of tree-level amplitude involving 2-graviton →2-graviton (hh→hh) scattering. Our analysis, particularly on reheating significantly reduces the parameter space of (m, β) for all models. The quadratic Gauss-Bonnet coupling parameter turns out to be more strongly constrained than that of the linear coupling. For the linear Gauss-Bonnet coupling function, we obtain β 103, with the condition β(m/MP)2≃10-4. However, the study of the Higgs inflation scenario in the presence of a Gauss-Bonnet term turns out to be completely disfavored.