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 decay of the Higgs boson as a representative case, showing how it yields a testable gravitational wave spectrum by future observations.