On the viability of m**2 phi**2 and natural inflation

TL;DR

This paper demonstrates that embedding quadratic and natural inflation models into a two-field framework with a super massive field can reconcile these models with Planck data by modifying the effective potential and reducing the inflaton's sound speed.

Contribution

It introduces a two-field inflation model with a super massive field that stabilizes the radial component and adjusts the inflationary predictions to match observations.

Findings

Two-field models improve agreement with Planck data.

The super massive field reduces the inflaton's sound speed.

Natural inflation models become consistent with observations.

Abstract

In the context of single field inflation, models with a quadratic potential and models with a natural potential with subplanckian decay constant are in tension with the Planck data. We show that, when embedded in a two-field model with an additional super massive field, they can become consistent with observations. Our results follow if the inflaton is the phase of a complex field (or an angular variable) protected by a mildly broken U(1) symmetry, and the radial component, whose mass is much greater than the Hubble scale, is stabilized at subplanckian values. The presence of the super massive field, besides modifying the effective single field potential, causes a reduction in the speed of sound of the inflaton fluctuations, which drives the prediction for the primordial spectrum towards the allowed experimental values. We discuss these effects also for the linear potential, and show that this model increases its agreement with data as well