Origin of ultra-light fields during inflation and their suppressed
  non-Gaussianity
  We study the structure of multi-field inflation models where the primordial
curvature perturbation is able to vigorously interact with an ultra-light
isocurvature field -- a massless fluctuation orthogonal to the background
inflationary trajectory in field space. We identify a class of inflationary
models where ultra-light fields can emerge as a consequence of an underlying
"scaling transformation" that rescales the entire system's action and keeps the
classical equations of motion invariant. This scaling invariance ensures the
existence of an ultra-light fluctuation that freezes after horizon crossing. If
the inflationary trajectory is misaligned with respect to the scaling symmetry
direction, then the isocurvature field is proportional to this ultra-light
field, and becomes massless. In addition, we find that even if the isocurvature
field interacts strongly with the curvature perturbation --transferring its own
statistics to the curvature perturbation-- it is unable to induce large
non-Gaussianity. The reason is simply that the same mechanism ensuring a
suppressed mass for the isocurvature field is also responsible for suppressing
its self-interactions. As a result, in models with light isocurvature fields
the bispectrum is generally expected to be slow-roll suppressed, but with a
squeezed limit that differs from Maldacena's consistency relation.
