Very Large Primordial Non-Gaussianity from multi-field: Application to   Massless Preheating
  In this paper we derive a generic expression, which is valid for scales larger than Hubble radius and contains only the local terms, for the second order curvature perturbations for more than one field, provided the expansion is sourced by the energy density of a single field. As an application, motivated by our previous paper [1], we apply our formalism to two fields during preheating, where the inflaton oscillations are sourced by $\lambda\phi^4$ potential which is governing the expansion of the Universe. A second field $\sigma$, coupled to the inflaton through $g^2\phi^2\sigma^2$, is excited from the vacuum fluctuations. The excited modes of $\sigma$ amplify the super-Hubble isocurvature perturbations, which seed the second order curvature perturbations to give rise to a significantly large non-Gaussianity. Our results show that within 3 inflaton oscillations for a range of parameters, $1< g^2/\lambda < 3$, the non-Gaussianity parameter becomes: $f_{NL}\geq {\cal O}(1000)$, which is already ruled out by the current WMAP observation. 
Asko Jokinen and Anupam Mazumdar