Electroweak Baryogenesis in Non-minimal Composite Higgs Models

TL;DR

This work argues that non-minimal composite Higgs models featuring a light gauge-singlet scalar can realize electroweak baryogenesis by combining a tree-level barrier that drives a strong first-order phase transition with CP-violating interactions in the top sector mediated by a dimension-five operator coupling the singlet to the Higgs. The authors show that a space-time varying CP phase in the top mass across the bubble wall generates the required BA, and that the resulting asymmetry can match observations for natural parameter choices while remaining consistent with EDM, LEP, and EWPO constraints. They analyze both spontaneous and explicitly broken CP-violating scenarios, provide mean-field and transport-equation treatments of the phase transition, and give benchmark points demonstrating viable EWBG with f in the multi-TeV range. The study highlights distinctive experimental signatures, notably in EDMs and Higgs-singlet mixing, offering tangible tests for this mechanism in current and future experiments.

Abstract

We address electroweak baryogenesis in the context of composite Higgs models, pointing out that modifications to the Higgs and top quark sectors can play an important role in generating the baryon asymmetry. Our main observation is that composite Higgs models that include a light, gauge singlet scalar in the spectrum [as in the model based on the symmetry breaking pattern SO(6)/SO(5)], provide all necessary ingredients for viable baryogenesis. In particular, the singlet leads to a strongly first-order electroweak phase transition and introduces new sources of CP violation in dimension-five operators involving the top quark. We discuss the amount of baryon asymmetry produced and the experimental constraints on the model.

Figures (4)

• Figure 1: The change in $\Theta_t$ needed during the EWPhT to reproduce the observed baryon asymmetry $(n_B-n_{\bar{B}})/n_\gamma\approx 6\times 10^{-10}$, as a function of the strength of the phase transition $v_c/T_c$. In the top plot the wall thickness $L_w$ is fixed in units of the critical temperature $T_c$ while in the bottom plot it is fixed in units of the critical VEV $v_c$.
• Figure 2: Shaded region: for $f/b=500\,\mathrm{GeV}$, $m_h=120\,\mathrm{GeV}$ and $m_s=80,\ 130\,\mathrm{GeV}$ (upper and lower plots), the $\Delta\Theta_t$ achieved for a given $v_c/T_c$ in the $Z_2$-symmetric case (a tiny explicit breaking is assumed, see Section \ref{['sec:SCPV']}). The black lines (dotted, dot-dashed, dashed, solid, double dashed-dotted) correspond to explicit examples with fixed $\lambda_m=0.25,0.5,0.75,1,1.5$, respectively. Points on the red lines match the observed baryon asymmetry (solid) or 1.5 (dotted), 0.75 (dashed) times that value. The vertical line marks $v_c/T_c=1$, below which the asymmetry would be erased by active sphalerons.
• Figure 3: Diagram illustrating the largest contribution to the electron EDM: the dashed line indicates a Higgs that mixes with the singlet, which then couples with the top.
• Figure 4: Bounds on the mass $m_2$ of the mostly-singlet mass eigenstate, and the mixing angle $\theta$ (shown only in the range $0$ to $\pi/4$ since the limits do not depend on the sign of $\theta$). EWPO exclude, for $m_1=120,\ 140\,\mathrm{GeV}$ (upper and lower plot respectively), the red region. Direct searches at LEP exclude the uniform turquoise region for light $m_2$. EDMs, for $f/b=500\,\mathrm{GeV}$ exclude the striped region [the uncertainty (\ref{['constraint_b']}) in the boundary of this region is reflected in the two limiting solid lines shown].