Gravitational waves from supermassive right-handed neutrinos produced at preheating

The postinflationary production of supermassive particles can have profound implications for the thermal history of the Universe and may leave observable imprints in the gravitational wave (GW) background. In scenarios where the inflaton couples predominantly to heavy fields, say right-handed neutrinos (RHNs), nonperturbative mechanisms such as parametric resonance can lead to their efficient production, even when their masses exceed the inflaton mass. Once produced, the RHNs emit gravitons through bremsstrahlung as they decay into the Standard Model (SM) particles via <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mi>N</a:mi> <a:mo stretchy="false">→</a:mo> <a:mo>ℓ</a:mo> <a:mo>+</a:mo> <a:mi>H</a:mi> </a:math> , enabled by the unavoidable minimal coupling to gravity, sourcing a stochastic GW background. We study this mechanism within the framework of <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"> <d:mi>α</d:mi> </d:math> -attractor inflationary models, highlighting how the resulting GW spectrum carries indirect imprints of the heavy sector and the postinflationary dynamics. This offers an observational window into otherwise inaccessible supermassive particles and provides a powerful probe of high-scale physics beyond the SM.