  We investigate loop corrections to the primordial fluctuations in the
single-field inflationary paradigm from spectator fields that experience a
smooth transition of their vacuum expectation values. We show that when the
phase transition involves a classical evolution effectively driven by a
negative mass term from the potential, important corrections to the curvature
perturbation can be generated by field perturbations that are frozen outside
the horizon by the time of the phase transition, yet the correction to tensor
perturbation is naturally suppressed by the spatial derivative couplings
between spectator fields and graviton. At one-loop level, the dominant channel
for the production of primordial fluctuations comes from a pair-scattering of
free spectator fields that decay into the curvature perturbations, and this
decay process is only sensitive to field masses comparable to the Hubble scale
of inflation.
