Generation of gravitational waves from symmetry restoration during
  inflation
  We discuss the possibility of a feature in the spectrum of inflationary
gravitational waves sourced by a scalar field $\chi$ whose vacuum fluctuations
are amplified by a rapidly time dependent mass. Unlike previous work which has
focused on the case in which the mass of the field $\chi$ vanishes only for an
instant before becoming massive again, we study a system where the scalar field
becomes and remains massless through the end of inflation as the consequence of
the restoration of a shift symmetry. After applying appropriate constraints to
our parameters, we find, for future CMB experiments, a small contribution to
the tensor-to-scalar ratio which can be at most of the order $r \sim 10^{-5}$.
At smaller scales probed by gravitational interferometers, on the other hand,
the energy density in the gravitational waves produced this way might be above
the projected sensitivity of LISA, $\Omega_{GW}\,h^2 \sim 10^{-13}$, in a
narrow region of parameter space. If there is more than one $\chi$ species,
then these amplitudes are enhanced by a factor equal to the number of those
species.
