A Diagrammatic Approach to Scalar Field Correlators during Inflation
  We consider a self-interacting scalar field in a de Sitter background and deal with the associated infrared divergences in a purely diagrammatic way using the in-in formalism. In the particular case of a large N O(N) invariant scalar field theory with quartic self-interactions we recover the result that the connected four-point correlation function, which is a signal of non-Gaussianity, is non-perturbatively enhanced with respect to its tree-level value. 
G. Petri