Probing the Electroweak Phase Transition with Higgs Factories and Gravitational Waves

TL;DR

This work investigates whether the electroweak phase transition (EWPT) can be strongly first order in minimal beyond-Standard-Model scenarios. It analyzes four model classes—Real Scalar Singlet, Stop-Like Scalar Doublet, Heavy Chiral Fermions, and Varying Yukawa Couplings—assessing how they generate a barrier in the Higgs potential via tree-level, radiative, or thermal effects and how they influence collider observables such as $\delta g_{hZZ}$, $\Delta\Gamma_{h\to\gamma\gamma}$, and $\lambda_3$, as well as the gravitational-wave signal. The results show that most first-order EWPT realizations predict sizable Higgs coupling deviations detectable by proposed Higgs factories, while space-based GW detectors like eLISA can probe only the strongest transitions with large $v(T)/T$. The study maps the model spaces of these scenarios, illustrating the complementary reach of precision collider measurements and gravitational-wave astronomy for elucidating EW symmetry breaking and early-universe dynamics.

Abstract

After the discovery of the Higgs boson, understanding the nature of electroweak symmetry breaking and the associated electroweak phase transition has become the most pressing question in particle physics. Answering this question is a priority for experimental studies. Data from the LHC and future lepton collider-based Higgs factories may uncover new physics coupled to the Higgs boson, which can induce the electroweak phase transition to become first order. Such a phase transition generates a stochastic background of gravitational waves, which could potentially be detected by a space-based gravitational wave interferometer. In this paper, we survey a few classes of models in which the electroweak phase transition is strongly first order. We identify the observables that would provide evidence of these models at the LHC and next-generation lepton colliders, and we assess whether the corresponding gravitational wave signal could be detected by eLISA. We find that most of the models with first order electroweak phase transition can be covered by the precise measurements of Higgs couplings at the proposed Higgs factories. We also map out the model space that can be probed with gravitational wave detection by eLISA.

Figures (6)

• Figure 1: Parameter space scan for the singlet model of Sec. \ref{['sec:Singlet']}. An orange point indicates a first order phase transition, a blue point indicates a strongly first order phase transition (\ref{['eq:washout_crit']}), and a green point indicates a very-strong first order phase transition with potentially detectable gravitational wave signal at eLISA. The right panels shows the predicted gravitational wave spectrum today along with the projected sensitivity of eLISA Caprini:2015zlo.
• Figure 2: Parameter space scan for the singlet model of Sec. \ref{['sec:Z2']} where a discrete $\mathbb{Z}_2$ symmetry forbids the Higgs-singlet mixing and suppresses the BSM modification to the $hZZ$ coupling.
• Figure 3: Parameter space scan for the singlet model of Sec. \ref{['sec:Unmixed']} where the Higgs-singlet mixing it tuned to zero.
• Figure 4: Parameter space scan for the stop-like model of Sec. \ref{['sec:Stop_Like']}. The projected sensitivity of figure Higgs factories (CEPC, ILC-500, FCC-ee) is sufficient to test the entire region of parameter space where the phase transition is first order (orange, blue, & green points).
• Figure 5: Results for the heavy chiral fermion model of Sec. \ref{['sec:Heavy_Fermions']}. The Higgs-to-diphoton decay rate is shown as a function of the chargino mass. We fix the Higgs-bino-Higgsino Yukawa coupling $h^{\prime} = 0$ and we show various values of the Higgs-wino-Higgsino Yukawa coupling $h$. Thick lines indicates parameters with a strongly first order phase transition (\ref{['eq:washout_crit']}), dashed lines indicate a weakly first order transition, and thin lines indicate a cross-over or second order transition.