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.
