The curvature perturbation in the axion-type curvaton model

Abstract

We study the axion-type curvaton model, with emphasis on the largefield regime where analytic results are very difficult to obtain. Weevaluate the tensor-scalar ratio r using WMAP normalization andthe non-linearity parameters fNL and gNL by solving theequations numerically using the δN formalism. We comparethem with results for the curvaton with quadratic potential. We findthat r is much smaller for the axion-type case compared to theresult from the quadratic potential, with the differenceincreasingly more pronounced at larger field values. gNL isfound to be positive, unlike the quadratic case where it isnegative, and the amplitude of gNL is much larger. Moreover,there is a nearly linear scaling between gNL and fNL, withsmall deviation from linearity at large field values. The slopebetween gNL and fNL depends on the parameterscharacterizing the axion-type curvaton model. We further consider amixed scenario where both the inflaton and the curvaton contributeto the primordial power spectrum and the non-Gaussianity parametersare found to be much larger than those in the case with quadraticpotential.