  The inflaton field responsible for inflation may not be a canonical
fundamental scalar. It is possible that the inflaton is a composite of fermions
or it may have a decay width. In these cases the standard procedure for
calculating the power spectrum is not applicable and a new formalism needs to
be developed to determine the effect of short range interactions of the
inflaton on the power spectrum and the CMB anisotropy. We develop a general
formalism for computing the power spectrum of curvature perturbations for such
non-canonical cases by using the flat space K\"all\'en-Lehmann spectral
function in curved quasi-de Sitter space assuming implicitly that the
Bunch-Davis boundary conditions enforces the inflaton mode functions to be
plane wave in the short wavelength limit and a complete set of mode functions
exists in quasi-de Sitter space. It is observed that the inflaton with a decay
width suppresses the power at large scale while a composite inflaton's power
spectrum oscillates at large scales. These observations may be vindicated in
the WMAP data and confirmed by future observations with PLANCK.
