The Baryon Asymmetry from a Composite Higgs

TL;DR

The SM fails to produce the observed baryon asymmetry due to the lack of a strong first-order EWPT and insufficient CP violation. The authors study a four-dimensional composite Higgs framework where the Higgs is a PNGB and the confinement transition is mediated by a dilaton, treating the coupled Higgs-dilaton dynamics and the possible variation of Yukawa couplings during the transition. They find that the joint Higgs-dilaton dynamics can yield a strong first-order EWPT often with the confinement transition, and a CP-violating source from a varying top Yukawa that can generate the observed baryon asymmetry while respecting flavour constraints; the framework makes testable predictions including a light dilaton, Higgs-dilaton mixing effects, EDM constraints, and a stochastic gravitational-wave signal. The results provide a natural route to EW baryogenesis in composite Higgs models and offer multiple experimental avenues to test the scenario at colliders, EDM experiments, and gravitational-wave missions such as LISA.

Abstract

We study the nature of the electroweak phase transition (EWPT) in models where the Higgs emerges as a pseudo-Nambu-Goldstone boson of an approximate global symmetry of a new strongly-interacting sector confining around the TeV scale. Our analysis focusses for the first time on the case where the EWPT is accompanied by the confinement phase transition of the strong sector. We describe the confinement in terms of the dilaton, the pseudo-Nambu-Goldstone boson of spontaneously broken conformal invariance of the strong sector. The dilaton can either be a meson-like or a glueball-like state and we demonstrate a significant qualitative difference in their dynamics. We show that the EWPT can naturally be strongly first-order, due to the nearly-conformal nature of the dilaton potential. Furthermore, we examine the sizeable scale variation of the Higgs potential parameters during the EWPT. In particular, we consider in detail the case of a varying top quark Yukawa coupling, and show that the resulting CP violation is sufficient for successful electroweak baryogenesis. We demonstrate that this source of CP violation is compatible with existing flavour and CP constraints. Our scenario can be tested in complementary ways: by measuring the CP-odd top Yukawa coupling in electron EDM experiments, by searching for dilaton production and deviations in Higgs couplings at colliders, and through gravitational waves at LISA.

Figures (2)

• Figure 1: Transition trajectories for a meson dilaton, $m_{\chi}=700$ GeV, $N=3$. Solid lines show the tunnelling path to the release point, while dotted lines indicate the subsequent rolling trajectory towards the minimum of the potential at $T_n$, indicated by a bullet.
• Figure 2: Results for the meson dilaton. In the red dashed region, no viable EW minimum can be found or the Higgs-dilaton mixing leads to too large deviations in the Higgs couplings. In the blue dashed region, the baryon asymmetry is washed out after reheating. We also show the cutoff $m_* = g_* f$. The chosen mass range satisfies current experimental constraints Blum:2014jca. Left: Phase-transition strength $h[T_n]/T_n$. The baryon asymmetry for benchmark point a (b) is $| \eta_B| \times 10^{10}\sim$ 5--5.5 (4--4.5). Center: Average Higgs vev during the phase transition relative to the condensate scale today, $h_{\rm avg}/f$. Right: Imaginary part of the top Yukawa as a function of the present value of $y/g_*^{1/2}$ and its anomalous dimension $\gamma_y$ for $|\beta_{y}| = \gamma_y y$, $\arg \beta_y = 0.1$ and $y_{tL}=\sqrt g_*$. The current and near future experimental sensitivities correspond respectively to approximately $2\times 10^{-2}$Cirigliano:2016nyn and $2\times 10^{-4}$Kumar:2013qya. The green bullet indicates the values used for the left and centre plots.