Differentiating between Neutrino Seesaw Mechanisms with Gravitational Waves

Neutrino masses are not explained within the Standard Model of elementary particles. One of the most elegant extensions of the theory addressing this problem is the seesaw mechanism. Its simplest version (type I) requires the existence of new heavy right-handed neutrinos. The other two scenarios involve a new scalar triplet (type II) or new triplet fermions (type III). Focusing on models with gauged lepton number, I will demonstrate how gravitational wave experiments can be used to distinguish between those three seesaw mechanism types through the first order phase transition contributions. I will also discuss cases where a flat spectrum from a high scale seesaw can coexist with a first order phase transition contribution, leading to intriguing signatures which can be searched for in gravitational wave experiments such as Big Bang Observer, DECIGO, Cosmic Explorer, Einstein Telescope, and LISA. I will also touch upon the possibility of explaining the recent NANOGrav signal within this framework.