Gravitational waves from particle decays during reheating

Gravitational waves have become an irreplaceable tool for exploring the post-inflationary universe. Their cosmological and astrophysical origins have been attracting numerous attention. In this Letter, we point out a novel source of ultra-high frequency gravitational waves: the decay of particles produced during the reheating era. We highlight the decays of the inflaton and the Higgs bosons in the Standard Model and in the gauged B-L model as representative cases, showing how they yield a testable gravitational wave spectrum by future observations. We find that the peak frequency and the strain in the Higgs boson decay in the gauged B-L model, when the symmetry breaking scale is higher than the inflationary scale, can be greater than those in the inflaton decay scenario. Typically, the predicted spectrum lies beyond MHz in frequency and thus calls for non-interferometric experiments. Regardless of the shape of the spectrum, on the other hand, the total energy density of the produced gravitational waves can be reached by future Big Bang nucleosynthesis limits.