Natural Cold Baryogenesis from Strongly Interacting Electroweak Symmetry Breaking

TL;DR

The article argues that cold electroweak baryogenesis can arise naturally from a strongly coupled, nearly conformal electroweak sector that undergoes a strongly first-order phase transition. During bubble collisions, energy is efficiently transferred into the scalar sector, driving Higgs winding and nonzero Chern-Simons number in the presence of CP violation, while Standard Model baryon-number violation generates the asymmetry. CP violation is modeled by a dimension-six operator that can evade EDM constraints, allowing baryogenesis without high-temperature sphaleron washout if reheating keeps the temperature below the sphaleron freeze-out. The framework is deliberately model-independent and leverages TeV-scale conformal dynamics, inviting lattice simulations to quantify the nonperturbative dynamics and potentially linking to extra-dimensional holographic pictures.

Abstract

The mechanism of "cold electroweak baryogenesis" has been so far unpopular because its proposal has relied on the ad-hoc assumption of a period of hybrid inflation at the electroweak scale with the Higgs acting as the waterfall field. We argue here that cold baryogenesis can be naturally realized without the need to introduce any slow-roll potential. Our point is that composite Higgs models where electroweak symmetry breaking arises via a strongly first-order phase transition provide a well-motivated framework for cold baryogenesis. In this case, reheating proceeds by bubble collisions and we argue that this can induce changes in Chern-Simons number, which in the presence of new sources of CP violation commonly lead to baryogenesis. We illustrate this mechanism using as a source of CP violation an effective dimension-six operator which is free from EDM constraints, another advantage of cold baryogenesis compared to the standard theory of electroweak baryogenesis. Our results are general as they do not rely on any particular UV completion but only on a stage of supercooling ended by a first-order phase transition in the evolution of the universe, which can be natural if there is nearly conformal dynamics at the TeV scale. Besides, baryon-number violation originates from the Standard Model only.

Figures (5)

• Figure 1: In standard EW baryogenesis, baryon number violation occurs via high temperature induced sphaleron transitions, along the "vacua" line $N_H = N_{CS}$. In contrast, in cold baryogenesis, sphaleron transitions are switched off and baryon number violation takes place in a two-step process via the production and decay of textures (configurations having $N_H \neq N_{CS}$). First, large kinetic energy stored in the scalar sector induces Higgs winding transitions. Second, these winding configurations can decay by changing Chern-Simons number, thus producing baryon number.
• Figure 2: The path of the scalar field for the three different potentials a), b), c) discussed in the text. "1" denotes the path in the expanding bubble walls. "2" is the path during the collision. "3" is the path in the collided region.
• Figure 3: Collision of planar bubble walls (with initial velocity $v_w=0.5$) using the potential (\ref{['eq:toy_pot']}) for $\lambda=1$. The left (right) plots are respectively for the nearly symmetric ($\eta=0.2$) and asymmetric ($\eta=0.6$) potentials. In case (b), the walls are reflected, and eventually stop expanding until the symmetric phase collapses again. In case (c) the field cannot leave the basin of attraction of the broken phase. These plots correspond to different slices of the collisions shown in Fig. \ref{['fig:coll']}.
• Figure 4: Collision of planar bubble walls for the potential (\ref{['eq:toy_pot']}) with $\lambda=1$. The top (bottom) plots use as initial wall velocity $v_w=0.5\, (0.98)$, respectively. The left (right) plots are for the symmetric (asymmetric) potential with $\eta=0.2 \, (0.6)$. Light (dark) gray corresponds to the broken (symmetric) phase. In the left case, the walls are reflected, and eventually stop expanding until the symmetric phase collapses again. In the right case, the field cannot leave the basin of attraction of the broken phase. The last pair of plots shows the time evolution of the fractions of the total energy in potential energy, bubble wall energy and "kinetic" energy of the classical scalar field in the case $v_w=0.5$ (see text).
• Figure 5: 3D plot corresponding to the bottom right plot of Fig. \ref{['fig:coll']}.