Induced-gravity inflation in no-scale supergravity and beyond

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 _n symmetry. We select two types of logarithmic Kähler potentials, one associated with a no-scale-type SU(2,1)/SU(2)× U(1)_R×_n Kähler manifold and one more generic. In both cases, imposing a lower bound on the parameter c involved in the coupling between the inflaton and the Ricci scalar curvature — e.g. c≳ 76, 105, 310 for n=2,3 and 6 respectively —, inflation can be attained even for subplanckian values of the inflaton while the corresponding effective theory respects the perturbative unitarity. In the case of no-scale SUGRA we show that, for every n, the inflationary observables remain unchanged and in agreement with the current data while the inflaton mass is predicted to be 3·10¹³ GeV. Beyond no-scale SUGRA the inflationary observables depend mildly on n and crucially on the coefficient involved in the fourth order term of the Kähler potential which mixes the inflaton with the accompanying non-inflaton field.