Electroweak Phase Transition, Gravitational Waves and Collider Probes in Multi-Scalar Dark Matter Scenarios

TL;DR

This paper explores multi-scalar dark matter models that enable strong electroweak phase transitions, producing detectable gravitational waves and offering new collider probe opportunities, extending beyond minimal scalar extensions.

Contribution

It introduces multi-scalar extensions of the Standard Model that allow larger Higgs portal couplings and stronger phase transitions, enhancing gravitational wave signals compared to single-scalar models.

Findings

Multi-scalar models can satisfy relic density with larger portal couplings.

Extended scalars induce strong first-order electroweak phase transitions.

Gravitational waves from these transitions could be observed by future detectors.

Abstract

We study scalar singlet extensions of the Standard Model (SM), focusing on scenarios where dark matter (DM) is stabilized by a \(\mathbb{Z}_2\) symmetry. In the minimal single-scalar extension of the SM, only a narrow region near the Higgs resonance remains viable, requiring small portal couplings in order to simultaneously satisfy the observed relic abundance and comply with the most recent direct detection limits from the LUX-ZEPLIN (LZ-2024) and XENON1T experiments. To address this limitation, we extend the dark sector by introducing additional real singlet scalars. In both two- and three-singlet extensions, we demonstrate that the observed dark matter relic density can be accommodated with larger Higgs portal couplings. These couplings significantly impact early-Universe dynamics by enhancing the strength of the electroweak phase transition. Both the two- and three-singlet scalar extensions can induce a strong first-order electroweak phase transition, generating stochastic gravitational waves potentially observable at future space-based detectors such as LISA and DECIGO. Notably, the three-singlet scenario induce an even stronger transition compared to the two-singlet case, enhancing the gravitational wave signal strength. Our results highlight the potential of extended scalar sectors as testable frameworks connecting dark matter and gravitational wave signals.