Large Nongaussianity in Axion Inflation

Abstract

The inflationary paradigm has enjoyed phenomenological success, however, a compelling particle physics realization is still lacking. The key obstruction is that the requirement of a suitably flat scalar potential is sensitive to Ultra-Violet (UV) physics. Axions are among the best-motivated inflaton candidates, since the flatness of their potential is naturally protected by a shift symmetry. We re-consider the cosmological perturbations in axion inflation, consistently accounting for the coupling to gauge fields \phi F \tilde{F}, which is generically present in these models. This coupling leads to production of gauge quanta, which provide a new source of inflaton fluctuations, \delta\phi. For an axion decay constant < 10^{-2} M_p, this effect typically dominates over the standard fluctuations from the vacuum, and saturates the current observational bounds on nongaussianity of the CMB anisotropies. Since sub-Planckian values of the decay constant are typical for concrete realizations that admit a UV completion (such as N-flation and axion monodromy), we conclude that large nongaussianity is easily obtained in very minimal and natural realizations of inflation.