Primordial Trispectrum from Isocurvature Fluctuations
  We study non-Gaussianity generated by adiabatic and isocurvature primordial
perturbations. We first obtain, in a very general setting, the non-linear
perturbations, up to third order, for an arbitrary number of cosmological
fluids, going through one or several decay transitions. We then apply this
formalism to the mixed curvaton and inflaton model, allowing for several decay
channels. We compute the various contributions to the bispectrum and
trispectrum resulting from adiabatic and isocurvature perturbations, which are
correlated in general. By investigating some hybrid decay scenario, we show
that significant non-Gaussianity of adiabatic and isocurvature types can be
generated without conflicting with the present isocurvature constraints from
the power spectrum. In particular, we find cases where non-Gaussianity of
isocurvature origin can dominate its adiabatic counterpart, both in the
bispectrum and in the trispectrum.
