Revisiting Singlet Fermion Dark Matter with a Scalar Portal: Connecting Higgs Phenomenology and Strong Electroweak Phase Transition

TL;DR

This work addresses the tension between achieving a strongly first-order electroweak phase transition and satisfying dark-matter and collider constraints by introducing a real singlet scalar s that does not acquire a zero-temperature VEV, coupled to a singlet Dirac fermion χ. The Higgs–singlet mixing is controlled by a separate trilinear portal μ_hs, while the Higgs-portal quartic λ_hs drives the finite-temperature potential, allowing a robust SFOEWPT without conflicting with direct-detection bounds. Dark matter phenomenology is set by the Yukawa coupling gχ and the mixing sin θ, yielding viable relic density through Higgs-funnel, heavy-scalar resonance, or degenerate mechanisms, with direct-detection rates exhibiting destructive interference (a blind-spot) between h1 and h2 exchanges. The model also predicts stochastic gravitational waves from the EWPT that can fall within the sensitivity of future space-based detectors, establishing a testable link between collider signatures, DM, and cosmology in a renormalizable framework. Overall, the paper demonstrates a cohesive scenario where collider, DM, and gravitational-wave probes jointly explore the scalar sector responsible for EWPT dynamics.

Abstract

We investigate a minimal extension of the Standard Model with a real singlet scalar and a singlet Dirac fermion acting as dark matter. Unlike a conventional singlet scalar setup, we assume that the singlet scalar does not acquire a vacuum expectation value at zero temperature. This decouples the scalar mixing angle from the Higgs-portal quartic coupling responsible for the strong first-order electroweak phase transition, allowing it to coexist with current collider and direct-detection constraints. The Higgs-singlet mixing is generated independently through a trilinear portal interaction. We check theoretical consistency conditions, various LHC limits on heavy scalar resonances, dark matter relic abundance, and direct detection bounds to delineate the viable parameter space. We perform a state-of-the-art analysis of the electroweak phase transition, which is shown to be achieved for a few benchmark points. We further compute the resulting stochastic gravitational wave spectra and find that several scenarios yield signals potentially observable at future space-based interferometers. Our results establish a unified and testable framework that connects collider phenomenology, first-order electroweak phase transition, and the resulting production of gravitational waves, along with the dark matter phenomenology, all within a simple renormalizable extension of the Standard Model.

Figures (8)

• Figure 1: Predicted cross section $\sigma(pp\to h_{2})\times \text{BR}(h_{2}\to h_{1}h_{1})$ in the $(m_{h_{2}},\sin\theta)$ plane. The solid curve shows the ATLAS 95% CL upper limit; the region above the curve is excluded.
• Figure 2: Exclusion in the $ZZ$ final state. The $ZZ\to 4\ell$ channel provides the strongest constraint across most of the mass range. Constraints from the $WW$ final state. The sensitivity is weaker than $ZZ$ due to larger backgrounds, but remains important for intermediate masses.
• Figure 3: Variation of relic abundance with dark matter mass $m_\chi$ with two different chosen values of $\sin \theta$ and $g_{\chi}$ mentioned in the inset. Left panel: $m_{h_2}=200$ GeV, $\lambda_{hs}=2.0$, Right panel: $m_{h_2}=350$ GeV, $\lambda_{hs}=4.0$. The black dashed line indicates the observed dark matter relic abundance, $\Omega_{\rm DM} h^2 = 0.12 \pm 0.001$Planck:2018vyg.
• Figure 4: Variation of the Yukawa coupling $g_\chi$ with DM mass $m_\chi$ maintaining relic abundance in the range $0.1 \le \Omega_{\rm DM} h^2 \le 0.121$. The color gradient represents the mixing angle $\sin\theta$. Left for $m_{h_2}=200$ GeV and $\lambda_{hs}=$ 2.0. Right: $m_{h_2}=350$ GeV and $\lambda_{hs}=4.0$.
• Figure 5: Variation of mixing angle $\sin\theta$ with DM mass $m_\chi$ maintaining relic abundance in the range $0.1\le\Omega_{\rm DM} h^2\le 0.121$. The color gradient represents the Yukawa coupling $g_\chi$. Left for $m_{h_2}=200$ GeV and $\lambda_{hs}=$ 2.0. Right: $m_{h_2}=350$ GeV and $\lambda_{hs}=4.0$.