Higgs Inflation in f(\Phi,R) Theory

We generalize the scalar-curvature coupling model ${\xi \Phi^2 R}$ of Higgs inflation to ${\xi \Phi^a R^b}$ to study inflation. We compute the amplitude and spectral index of curvature perturbations generated during inflation and fix the parameters of the model by comparing these with the Planck$+$WP data. We find that if the scalar self coupling $\lambda$ is in the range $(10^{-5}-0.1)$, parameter $a$ in the range $(2.3 -3.6)$ and $b$ in the range $(0.77-0.22)$ at the Planck scale, one can have a viable inflation model even for $\xi \simeq 1$. The tensor to scalar ratio $r$ in this model is small and our model with scalar-curvature couplings is not ruled out by observational limits on $r$ unlike the pure $\frac{\lambda}{4} \Phi^4$ theory. By requiring the curvature coupling parameter to be of order unity, we have evaded the problem of unitarity violation in scalar-graviton scatterings which plague the $\xi \Phi^2 R$ Higgs inflation models. We conclude that the Higgs field may still be a good candidate for being the inflaton in the early universe if one considers higher dimensional curvature coupling.