Effective Field Theory and Decoupling in Multi-field Inflation: An
  Illustrative Case Study
  We explore the effects of heavy degrees of freedom on the evolution and
perturbations of light modes in multifield inflation. We use a simple two-field
model as an example to illustrate the subtleties of integrating out massive
fields in a time-dependent background. We show that when adiabaticity is
violated due to a sharp turn in field space, the roles of massive and massless
field are interchanged, and furthermore the fields are strongly coupled; thus
the system cannot be described by an effective single field action. Further
analysis shows that the sharp turn imparts a non Bunch-Davis component in each
perturbation mode, leading to oscillatory features in the power spectrum, and a
large resonantly enhanced bispectrum.
