Boiling away asymmetries: low-scale phase transitions, gravitational waves and leptogenesis

TL;DR

This paper explores how supercooled phase transitions and resulting entropy dilution impact leptogenesis, linking gravitational wave signals with neutrino mass models and constraining viable baryogenesis scenarios.

Contribution

It demonstrates that entropy dilution from first-order phase transitions can significantly restrict leptogenesis parameter space, connecting gravitational wave observations with baryogenesis models.

Findings

Leptogenesis parameter space is reduced by entropy dilution effects.

SGWB signals can exclude leptogenesis scenarios testable by future experiments.

Phenomenological link established between gravitational waves, phase transitions, and neutrino masses.

Abstract

Leptogenesis is one of the most popular mechanisms to account for the observed baryon asymmetry of the Universe. A generic feature of leptogenesis is a large separation of scales between the epoch of baryon asymmetry production (sphaleron freeze-out at temperature $T \sim 130$ GeV) and the one where it affects the big bang nucleosynthesis processes (BBN at $T \sim 1$ MeV). Any entropy release between these two epochs would lead to a dilution of previously produced relics, such as the baryons. Motivated by the recent evidence of a stochastic gravitational waves background (SGWB) in the nHz frequency range, we consider the case of supercooled first-order phase transition and we study the impact of the induced entropy dilution on the leptogenesis parameter space. We employ the Type-I seesaw with 2 right-handed neutrinos as benchmark scenario, and demonstrate that the viable leptogenesis parameter space is significantly reduced. Interestingly, the values of dilution predicted by the SGWB best fit points in several first order phase transition scenarios would completely exclude the leptogenesis parameter space testable by future experiments, thus establishing a phenomenological interconnection between leptogenesis, SGWB, first order phase transition and neutrino mass generation. Our analysis can be generalised to different leptogenesis models and entropy dilution mechanisms.