Large Nongaussianity in Axion Inflation
  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.
