Non-gaussianity in axion N-flation models Quadratic and $\lambda\phi^4$   plus axion potentials
  In this paper we investigate large non-gaussianity in axion N-flation models, taking account while dynamically a large number of axions begin away from the hilltop region(come down from the hill) and so serve only to be the source of the Hubble rate. Therefore the single field stays closest to the hilltop sources the non-Gaussianity. In this case most of axions can be replaced by a single effective field with a quadratic potential. So our potential will contain two fields. The full cosine is responsible for the axion closest to hilltop and quadratic term which is a source for Hubble rate [4]. We obtain power spectrum, spectral index and non-gaussianity parameter, then we impose conditions from WMAP for power spectrum and spectral index and see how large on non-gaussianity parameter it is possible to achieve with such conditions. Finally we swap quadratic term to {\lambda}{\phi}^4 and see whether this makes it harder or easier to achieve large non-gaussianity.We find large non-gaussianity is achievable by imposing data from WMAP conditional on axion decay constant f has reasonable value in connection with Planck mass and by requiring number of e-folds must be bounded between 40-60.When we swap to {\lambda}{\phi}^4 we find that it is harder to achieve non-Gaussianity, because we are imposed to investigate only {\lambda}{\phi}^4 domination in consistency with WMAP data for spectral index. Although, in this case we find that large non-gaussianity is still achievable. Finally we verify imposing the condition for spectral index to be nearly one and find acceptable and detectable value for non-gaussianity typically of order of 10 and 100 depending on value of decay constant f.Swapping to {\lambda}{\phi}^4 in this case does not give us any significant relation. In this paper we consider models which can generate large non-gaussianity. We restrict ourselves to axion N-flation models. 
Mehran Kamarpour
gr-qc astro-ph.CO