Gravitational wave, collider and dark matter signals from a scalar singlet electroweak baryogenesis

TL;DR

This paper investigates a minimal scalar singlet extension of the Standard Model that couples to the Higgs to enable electroweak baryogenesis, while also providing a dark matter candidate. By analyzing the vacuum structure, phase-transition dynamics, and one-loop corrections, it shows how a strong first-order EWPT can arise and produce observable gravitational waves, with collider probes offering complementary reach. The study also assesses dark matter constraints, finding that EWBG-favored regions typically correspond to subdominant DM and are tightly constrained by direct detection; a cosmological modification can relax certain bounds but does not dramatically expand viable parameter space without additional dark-sector states. Overall, gravitational wave observations emerge as a particularly sensitive probe of EWBG in this scenario, with significant implications for future multi-messenger searches and model-building directions.

Abstract

We analyse a simple extension of the SM with just an additional scalar singlet coupled to the Higgs boson. We discuss the possible probes for electroweak baryogenesis in this model including collider searches, gravitational wave and direct dark matter detection signals. We show that a large portion of the model parameter space exists where the observation of gravitational waves would allow detection while the indirect collider searches would not.

Figures (7)

• Figure 1: Parameter space of the scalar singlet model relevant for EWBG. The yellow region is excluded because in that region, the electroweak minimum is not the global minimum at zero temperature. The blue region realises a strong first-order phase transition whereas the light blue region can still be allowed due to the cosmological modification. The solid red line marks the boundary between the regions where $\mu_S^2 (T_c) < 0$ and $\mu_S^2 (T_c) > 0$ (see text for more details).
• Figure 2: Parameter space of the scalar singlet model relevant for EWBG along with the reach of various collider experiments. The yellow shaded region is excluded because in that region, the electroweak minimum is not the global minimum at zero temperature. In the grey region, the universe is trapped in a metastable vacuum that preserves electroweak symmetry. The blue region realises a strong first-order phase transition whereas the light blue region can still be allowed due to the cosmological modification. Regions above the dotted and dashed lines will be accessible at colliders. Here $\Delta \lambda_3 \equiv (\lambda^{\textrm{SM}}_{3} - \lambda_3)/\lambda^{\textrm{SM}}_{3}$ is the modification of the triple Higgs coupling with respect to the SM.
• Figure 3: Parameter space of the scalar singlet model relevant for EWBG. In the green and purple regions GW signals produced during the phase transition will be accessible in future experiments such as LISA and BBO respectively. A few example points are also highlighted to match with their GW spectra in Fig. \ref{['fig:GWplot']}.
• Figure 4: Spectra of GWs from the electroweak phase transition for a few example points that are also marked in Fig. \ref{['fig:GWdetectionplot']}. Projected sensitivities of the future based GW detectors such as LISA, BBO as well as current sensitivity of LIGO are also shown.
• Figure 5: Parameter space of the scalar singlet model relevant for EWBG together with the DM abundance and corresponding direct detection exclusion limits. Constraints from the vacuum structure of the theory are also taken into account, hence the reason why the abundance or the direct detection limits do not enter into the gray or yellow shaded regions.