Bubble-assisted Leptogenesis

TL;DR

This paper investigates how bubble-assisted first-order phase transitions can enable leptogenesis at lower right-handed neutrino masses, potentially reducing washout effects and producing observable gravitational wave signatures.

Contribution

It introduces a novel framework combining bubble dynamics with leptogenesis, demonstrating reduced RHN mass requirements and potential GW signals.

Findings

RHN mass bounds are lowered from 10^{11} GeV to 5×10^9 GeV due to bubble effects.

Lower RHN masses (~10^7 GeV) are achievable with bubble assistance, alleviating strong washout.

Potential gravitational wave signatures could be observable at terrestrial interferometers.

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 $M_N\sim 10^{11}\text{ GeV}$ without bubble assistance is now predicted at $M_N \sim 5\times 10^9 \text{ GeV}$ 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, $M_N \sim 10^{7}\text{ GeV}$, 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, $M_N \lesssim 5\times 10^9 \text{ GeV}$ may be probable.