Abstract
We show how the generation of right-handed neutrino masses in Majoron models may be associated with a first-order phase transition and accompanied by the production of a stochastic background of gravitational waves (GWs). We explore different energy scales with only renormalizable operators in the effective potential. If the phase transition occurs above the electroweak scale, the signal can be tested by future interferometers. We consider two possible energy scales for phase transitions below the electroweak scale. If the phase transition occurs at a GeV, the signal can be tested at LISA and provide a complementary cosmological probe to right-handed neutrino searches at the FASER detector. If the phase transition occurs below 100 keV, we find that the peak of the GW spectrum is two or more orders of magnitude below the putative NANOGrav GW signal at low frequencies, but well within reach of the SKA and THEIA experiments. We show how searches of very low frequency GWs are motivated by solutions to the Hubble tension in which ordinary neutrinos interact with the dark sector. We also present general calculations of the phase transition temperature and Euclidean action that apply beyond Majoron models.
Highlights
The recent discovery of gravitational waves (GWs) provides a powerful phenomenological tool to probe the imprint of new physics in the early universe
We show how the generation of right-handed neutrino masses in Majoron models may be associated with a first-order phase transition and accompanied by the production of a stochastic background of gravitational waves (GWs)
It is established that both electroweak symmetry breaking and the approximate QCD chiral symmetry breaking occur as a smooth crossover, with no phase transition, so that no associated stochastic GW background is predicted within the standard model (SM)
Summary
The recent discovery of GWs provides a powerful phenomenological tool to probe the imprint of new physics in the early universe. The necessity to extend the SM to explain outstanding cosmological puzzles and to incorporate neutrino masses and mixing, motivates the study of first-order phase transitions in the early universe In this way primordial GWs are an important tool, complementary to more traditional ones, to probe physics beyond the SM. The Majoron model [10] provides a way to embed the type-I seesaw mechanism and at the same time to generate Majorana masses This is done through the introduction of a complex scalar field with terms in the Lagrangian respecting U(1)L symmetry spontaneously broken below a critical temperature. Since spontaneous symmetry breaking can occur via a strong first-order phase transition, an associated production of GWs is possible, in a way that GW experiments can probe the origin of Majorana masses and give insight on the scale of symmetry breaking.
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