Phenomenology of a Pseudo-Scalar Inflaton: Naturally Large Nongaussianity

TL;DR

This paper explores how pseudo-scalar inflatons coupled to gauge fields naturally produce large, distinctive nongaussianity in cosmological perturbations, with observable implications for inflation models.

Contribution

It introduces a mechanism where gauge field production during inflation leads to significant nongaussianity, expanding understanding of inflationary perturbation sources.

Findings

Gauge field quanta are copiously produced by the inflaton.

Produced gauge quanta source highly nongaussian inflaton fluctuations.

The model predicts large nongaussianity with specific observational signatures.

Abstract

Many controlled realizations of chaotic inflation employ pseudo-scalar axions. Pseudo-scalars \phi are naturally coupled to gauge fields through c \phi F \tilde{F}. In the presence of this coupling, gauge field quanta are copiously produced by the rolling inflaton. The produced gauge quanta, in turn, source inflaton fluctuations via inverse decay. These new cosmological perturbations add incoherently with the "vacuum" perturbations, and are highly nongaussian. This provides a natural mechanism to generate large nongaussianity in single or multi field slow-roll inflation. The resulting phenomenological signatures are highly distinctive: large nongaussianity of (nearly) equilateral shape, in addition to detectably large values of both the scalar spectral tilt and tensor-to-scalar ratio (both being typical of large field inflation). The WMAP bound on nongaussianity implies that the coupling, c, of the pseudo-scalar inflaton to any gauge field must be smaller than about 10^{2} M_p^{-1}.