Nonperturbative production of massless scalars during inflation and generation of gravitational waves

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. 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 similar to 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) similar to 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.