The two-field regime of natural inflation

TL;DR

This paper investigates a two-field natural inflation model, demonstrating a regime where both fields actively influence inflation and align with observational data, especially considering mass hierarchies and isocurvature constraints.

Contribution

It introduces a detailed analysis of the two-field regime in natural inflation, including mass hierarchy effects and initial conditions, expanding the understanding of its observational viability.

Findings

Both fields can be active during inflation within observational bounds.

Mass hierarchy influences the inflation dynamics and predictions.

Predictions remain consistent with isocurvature bounds under certain conditions.

Abstract

The simplest two-field completion of natural inflation has a regime in which both fields are active and in which its predictions are within the Planck 1-$\sigma$ confidence contour. We show this for the original model of natural inflation, in which inflation is achieved through the explicit breaking of a U(1) symmetry. We consider the case in which the mass coming from explicit breaking of this symmetry is comparable to that from spontaneous breaking, which we show is consistent with a hierarchy between the corresponding energy scales. While both masses are comparable when the observable modes left the horizon, the mass hierarchy is restored in the last e-foldings of inflation, rendering the predictions consistent with the isocurvature bounds. For completeness, we also study the predictions for the case in which there is a large hierarchy of masses and an initial period of inflation driven by the (heavy) radial field.