Two-loop corrections to Starobinsky-Higgs inflation

Higgs inflation and $R^2$-inflation (Starobinsky model) are two limits of the same quantum model, hereafter called Starobinsky-Higgs. We analyse the two-loop action of the Higgs-like scalar $phi$ in the presence of: 1) non-minimal coupling ($xi$) and 2) quadratic curvature terms. The latter are generated at the quantum level with $phi$-dependent couplings ($tildealpha$) even if their tree-level couplings ($alpha$) are tuned to zero. Therefore, the potential always depends on both Higgs field $phi$ and scalaron $rho$, hence multi-field inflation is a quantum consequence. The effects of the quantum (one- and two-loop) corrections on the potential $hat W(phi,rho)$ and on the spectral index are discussed, showing that the Starobinsky-Higgs model is in general stable in their presence. Two special cases are also considered: first, for a large $xi$ in the quantum action one can integrate $phi$ and generate a refined Starobinsky model which contains additional terms $xi^2 R^2ln^p (xi vert Rvert/mu^2)$, $p=1,2$ ($mu$ is the subtraction scale). These generate corrections linear in the scalaron to the usual Starobinsky potential and a running scalaron mass. Second, for a small fixed Higgs field $phi^2 ll M_p^2/xi$ and a vanishing classical coefficient of the $R^2$-term, we show that the usual Starobinsky inflation is generated by the quantum corrections alone, for a suitable non-minimal coupling ($xi$).