Electroweak Phase Transition and Baryogenesis in Composite Higgs Models

TL;DR

<3-5 sentence high-level summary>Bruggisser et al. study how a composite Higgs scenario with a light dilaton can realize a strongly first-order electroweak phase transition and support electroweak baryogenesis. They develop a two-field zero- and finite-temperature EFT for the Higgs and dilaton, incorporating running Yukawa couplings that vary with the dilaton vev, and perform a full numerical tunnelling analysis to map viable regions. The work identifies CP-violating sources from varying charm and top mixings, showing that sufficient baryon asymmetry can be achieved in realistic parameter ranges, and discusses experimental probes including dilaton searches, Higgs/dilaton couplings, EDM constraints, and gravitational waves. The results motivate further UV-complete modeling and highlight distinctive collider and gravitational-wave signatures of this baryogenesis mechanism.

Abstract

We present a comprehensive study of the electroweak phase transition in composite Higgs models, where the Higgs arises from a new, strongly-coupled sector which confines near the TeV scale. This work extends our study in Ref. [1]. We describe the confinement phase transition in terms of the dilaton, the pseudo-Nambu-Goldstone boson of broken conformal invariance of the composite Higgs sector. From the analysis of the joint Higgs-dilaton potential we conclude that in this scenario the electroweak phase transition can naturally be first-order, allowing for electroweak baryogenesis. We then extensively discuss possible options to generate a sufficient amount of CP violation - another key ingredient of baryogenesis - from quark Yukawa couplings which vary during the phase transition. For one such an option, with a varying charm quark Yukawa coupling, we perform a full numerical analysis of tunnelling in the Higgs-dilaton potential and determine regions of parameter space which allow for successful baryogenesis. This scenario singles out the light dilaton region while satisfying all experimental bounds. We discuss future tests. Our results bring new opportunities and strong motivations for electroweak baryogenesis.

Figures (16)

• Figure 1: Sketch of different trajectories for the phase transition in composite Higgs models. The field $\chi$ sets the size of the strong-sector condensate and $h$ is the Higgs vev. The blue points correspond to (meta)stable vacua of the theory, and the black lines show possible phase transition trajectories.
• Figure 2: Examples of the running of the mixing parameters (\ref{['eq:solmixing']}) (the effective Yukawa couplings follow similar trends), for positive $\gamma$ and an order-one initial value (blue), a small negative $\gamma$ and a small initial value (green), a negative $\gamma$ and a small initial value (red), a negative $\gamma$ and an order-one initial value (yellow). The green area shows the range of energies relevant for our analysis. The behaviour of the mixings outside the green region is not relevant for our analysis and can differ from what we show. For instance, all the initial values could be of order one, but the sign of the anomalous dimension could change with energy.
• Figure 3: Examples of potentials with a valley along the direction $h=0$ (left plot) and with a valley along $h\sim\chi$ (right plot). The green line shows the $\chi$-dependent minimum of the Higgs potential. Since the Higgs potential is loop-suppressed with respect to the dilaton potential, and we assume order-one values of the mixings $y$, the valleys are not very pronounced.
• Figure 4: Same as in Fig. \ref{['fig:simppot']} but in canonical variables $\chi_{1,2}$.
• Figure 5: Schematic shape of the free energy as a function of $\chi$, in the "hot" region with $g_\chi \chi\lesssim T$ (in red) and in the "cold" region with $g_\chi \chi\gtrsim T$ (in blue).