A Second Higgs Doublet in the Early Universe: Baryogenesis and Gravitational Waves

TL;DR

The paper investigates whether simple Type II Two-Higgs-Doublet Models can generate the observed baryon asymmetry of the Universe via electroweak baryogenesis while remaining consistent with stringent ACME electron EDM bounds. It analyzes the electroweak phase transition dynamics in a hierarchical 2HDM spectrum, computes the bubble-wall properties and CP-violating sources, and solves transport equations to yield the BAU, identifying parameter windows that satisfy both EDM and flavor constraints. In parallel, the authors compute the resulting gravitational-wave spectrum from the strong first-order EWPT, showing that acoustic-wave sourced signals can fall within LISA’s sensitivity for a subset of viable benchmarks. The work demonstrates a consistent, testable link between early-Universe baryogenesis and gravitational-wave signatures in a minimal BSM framework, emphasizing the interplay between collider, EDM, and GW probes and outlining how future experiments could confirm or rule out this scenario.

Abstract

We show that simple Two Higgs Doublet models still provide a viable explanation for the matter-antimatter asymmetry of the Universe via electroweak baryogenesis, even after taking into account the recent order-of-magnitude improvement on the electron-EDM experimental bound by the ACME Collaboration. Moreover we show that, in the region of parameter space where baryogenesis is possible, the gravitational wave spectrum generated at the end of the electroweak phase transition is within the sensitivity reach of the future space-based interferometer LISA.

Figures (3)

• Figure 1: Phase transition strength (left) and wall thickness (right) as a function of fractional vacuum energy released in the plasma at nucleation temperature $T_n$ for the simplified toy model (dashed line), the corresponding 2HDM with $M=m_h/\sqrt{2}$, $\tan\beta=1$ and degenerate masses (green solid line) and the hierarchical case considered throughout this work (blue solid line). Also shown are the wall velocities for the toy model (dot-dashed line).
• Figure 2: EDM constraints for benchmarks described in text. The dash-dotted line corresponds to the eEDM bound before the ACME experiment. The black dashed lines correspond to the minimum CPV phase necessary for successful baryogenesis for $M=m_{H^0}=200$ GeV and varying $m_{A^0}=m_{H^\pm}$.
• Figure 3: Gravitational wave spectrum for differing values of $m_{A^0}=m_{H^\pm}$. The solid colored lines are the prospective sensitivity for different LISA configurations (see the text and ref. Caprini:2015zlo for more details).