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 beta, an inflationary analysis provides us a big cosmologically viable region in the space of (m, beta), where m is the mass of the inflaton. However, we study further on constraining beta 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, beta) 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 beta less than or similar to 10(3), with the condition beta(m/M-P)(2) similar or equal to 10(-4). However, the study of the Higgs inflation scenario in the presence of a Gauss- Bonnet term turns out to be completely disfavored.