When inverse seesaw meets inverse electroweak phase transition: a novel path to leptogenesis

TL;DR

This paper proposes a novel nonthermal leptogenesis mechanism driven by a two-stage electroweak history: an inverse EWPT triggered by a first generation of TeV-scale vectorlike leptons, followed by a direct EWPT. The second generation of vectorlike leptons realizes a minimal inverse seesaw, providing heavy neutrinos that participate in CP-violating decays once produced abundantly on expanding bubble walls. The mechanism leverages wall-frame lepton scattering to generate heavy neutrinos, with CP asymmetries arising from high-energy phases in the Yukawa sector, and uses EW sphalerons during the inverse EWPT to convert the lepton asymmetry into the observed BAU. The framework yields testable predictions for collider VLLs, Higgs diphoton rates, and potentially stochastic gravitational waves, offering a coherent path from beyond-Standard-Model flavor and mass generation to cosmological baryogenesis. The findings show that bubble-assisted leptogenesis can dominate over thermal leptogenesis in this setup and provide a concrete, experimentally accessible scenario for connecting neutrino mass generation with the early Universe dynamics.

Abstract

We propose a new nonthermal leptogenesis mechanism triggered by the cosmic first-order phase transition. The Standard Model is extended with two generations of TeV-scale vectorlike leptons. The lighter generation gives rise to an inverse electroweak phase transition of the Higgs field at $T\sim200~{\rm GeV}$, restoring the symmetry, and resulting in relativistic bubble expansion in the space. The heavier generation is responsible for neutrino masses via the inverse seesaw mechanism. The interaction between bubble walls and particles in the plasma abundantly produces the vectorlike leptons, and they subsequently undergo CP-violating decay to generate the baryon asymmetry. This mechanism is testable at current and future particle experiments.

Figures (5)

• Figure 1: Sketch of our mechanism. When the Universe cools down, the Higgs effective potential evolves and triggers two successive first-order EWPTs, first an inverse one and then a direct one. During the inverse EWPT, RHNs are produced via the scattering between SM light leptons $\ell_L$ and the bubble walls, and then decay immediately and generate the BAU. See the text for details.
• Figure 2: The inverse EWPT parameter space projected to two-dimensional plots. Left: the physical masses $m_{X_1}$ v.s. $m_{X_2}$. Right: the Yukawa couplings $y_{X_L}$ v.s. $y_{X_R}$. The black and dark red colors represent $X=N$ and $E$, respectively.
• Figure 3: The phase transition parameters for the inverse (left) and direct (right) EWPTs.
• Figure 4: The bubble-assisted leptogenesis parameter space projected to two-dimensional plots. Left: the physical masses $m_{X_1}$ v.s. $m_{X_2}$ with $X=N$ and $E$. Middle: the Majorana mass term $\mu_N$ v.s. the $\theta_R$ angle in CI parametrization. Right: the lepton-number-breaking Yukawa coupling $\lambda_N$ v.s. the decay asymmetries $\epsilon_1+\epsilon_2$ in the EW-symmetric vacuum.
• Figure 5: Left: the physical mass $m_{N_1}$ v.s. the $h\to\gamma\gamma$ signal strength deviation. The red points yield a SNR $>1$ at the GW detector BBO. Right: the physical mass $m'_{N_1}$ v.s. the decay asymmetries in the EW-breaking vacuum.