Bubble-assisted leptogenesis

Abstract

We explore the possibility of embedding thermal leptogenesis within a first-order phase transition (FOPT) such that RHNs remain massless until a FOPT arises. Their sudden and violent mass gain allows the neutrinos to become thermally decoupled, and the lepton asymmetry generated from their decay can be, in principle, free from the strong wash-out processes that conventional leptogenesis scenarios suffer from, albeit at the cost of new washout channels. To quantify the effect of this enhancement, we consider a simple setup of a classically scale-invariant B − L potential, which requires three RHNs with similar mass scales, in the “strong-washout” regime of thermal leptogenesis. Here we find that parameter space which requires M_N ~ 10¹¹ GeV without bubble assistance is now predicted at M_N ~ 5 × 10⁹ GeV suggesting a sizeable reduction from bubble effects. We numerically quantify to what extent such a framework can alleviate strong-washout effects and we find the lower bound on the RHN mass, M_N ~ 10⁷ GeV, below which bubble-assisted leptogenesis cannot provide an enhancement. We also study the signature possibly observable at GW terrestrial interferometers and conclude that bubble-assisted leptogenesis models with relatively light masses, M_N ≲ 5 × 10⁹ GeV may be probable.