Ultra-high frequency gravitational waves from scattering, Bremsstrahlung and decay during reheating

We investigate ultra-high frequency gravitational waves (GWs) from gravitons generated during inflationary reheating. Specifically, we study inflaton scattering with its decay product, where the couplings involved in this 2 -> 2 scattering are the same as those in the 1 -> 3 graviton Bremsstrahlung process. We compute the graviton production rate via such 2 -> 2 scattering. Additionally, we compare the resulting GW spectrum with that from Bremsstrahlung as well as that from pure 2 -> 2 inflaton scatterings. For completeness, the GW spectrum from graviton pair production through one-loop induced 1 -> 2 inflaton decay is also analyzed. With a systematic comparison among the four sources of GWs, we find that 2 -> 2 inflaton scattering with its decay product can dominate over Bremsstrahlung if the reheating temperature is larger than the inflaton mass. Pure inflaton 2 -> 2 scattering is typically subdominant compared to Bremsstrahlung except in the high-frequency tail. The contribution from one-loop induced 1 -> 2 inflaton decay is shown to be suppressed compared to Bremsstrahlung and pure inflaton 2 -> 2 scattering.