Induced-Gravity Inflation in Supergravity Confronted with Planck 2015 & BICEP2/Keck Array

Supersymmetric versions of induced-gravity inflation are formulated within Supergravity (SUGRA) employing two gauge singlet chiral superfields. The proposed superpotential is uniquely determined by applying a continuous R and a discrete Z_2 symmetry. We also employ a logarithmic Kahler potential respecting the symmetries above and including all the allowed terms up to fourth order in powers of the various fields. When the Kahler manifold exhibits a no-scale-type symmetry, the model predicts spectral index ns=0.963 and tensor-to-scalar r=0.004. Beyond no-scale SUGRA, ns and r depend crucially on the coefficient ksphi involved in the fourth order term, which mixes the inflaton Phi with the accompanying non-inflaton superfield S in the Kahler potential, and the prefactor encountered in it. Increasing slightly the latter above (-3), an efficient enhancement of the resulting r can be achieved putting it in the observable range favored by the Planck and BICEP2/Keck Array results. In all cases, imposing a lower bound on the parameter cR, involved in the coupling between the inflaton and the Ricci scalar curvature, inflation can be attained for subplanckian values of the inflaton while the corresponding effective theory respects the perturbative unitarity.