The Standard-smooth Variant of Hybrid Inflation

We consider the extension of the supersymmetric Pati-Salam model introduced in order to solve the b-quark mass problem in supersymmetric theories with Yukawa unification, universal boundary conditions and mu>0. This model naturally leads to the new shifted and new smooth hybrid inflation scenarios, which, however, yield, in minimal supergravity, too large values of the spectral index n_s. We show that this problem can also be resolved within the same model by a two-stage inflationary scenario based only on renormalizable superpotential interactions. The first stage is of the standard and the second of the new smooth hybrid type. The cosmological scales exit the horizon during the first stage of inflation and acceptable n_s's can be achieved by restricting the number of e-foldings of our present horizon during this inflationary stage. The additional e-foldings needed for solving the horizon and flatness problems are naturally provided by the second stage of inflation. Monopoles are formed at the end of the first stage of inflation and are, subsequently, diluted by the second stage of inflation so that their density in the present universe is utterly negligible.