Probing the Origin of Neutrino Masses via Gravitational Waves from Cosmic Strings

The Standard Model of elementary particles does not explain the origin of neutrino masses nor the domination of matter over antimatter in the Universe. One of the most elegant extensions of the theory providing solutions to those problems involves postulating the existence of heavy right-handed neutrinos and a new U(1) gauge symmetry, leading to the seesaw mechanism for neutrino masses. The breaking of this U(1) symmetry triggers the emission of gravitational waves in the early Universe via cosmic strings, domain walls, and first-order phase transitions, potentially detectable today. Focusing on the special case when the extra U(1) symmetry is either lepton number (L) or baryon minus lepton number (B-L), I will show how to probe different types of seesaw mechanisms (I, II, and III) in upcoming gravitational wave experiments. In particular, I will compare the expected cosmic strings signatures determined using the loop distribution functions from Blanco-Pillado et al. (2014) and Lorenz et al. (2010). If there are two U(1) symmetries at the high scale, one of which is broken via two scalars, a novel and unique gravitational wave signal may arise, involving a simultaneous presence of cosmic string and domain wall signatures. This type of signal has not been considered before, and can be searched for in near-future gravitational wave experiments like LISA, Big Bang Observer, DECIGO, Cosmic Explorer, and Einstein Telescope.