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.