HIGGS INFLATION IN f (Φ, R) THEORY

We generalize the scalar-curvature coupling model xi Phi R-2 of Higgs inflation to xi Phi R-a(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 similar or equal to 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 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 R-2 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.