Gravitational Waves and Dark Matter in the Gauged Two-Higgs Doublet Model

TL;DR

This paper explores the potential for detecting gravitational waves from a strong first-order electroweak phase transition in the gauged two-Higgs doublet model, which also provides a dark matter candidate, using future detectors and experiments.

Contribution

It demonstrates the viability of a two-step phase transition in G2HDM and predicts detectable gravitational wave signals, linking collider, gravitational wave, and dark matter detection prospects.

Findings

A strong first-order phase transition can occur in G2HDM.

Future detectors like LISA and BBO could observe the gravitational waves.

Dark matter detection experiments can also probe the model's parameter space.

Abstract

We investigate the possibility of a strong first-order electroweak phase transition during the early universe within the framework of the gauged two-Higgs doublet model (G2HDM) and explore its detectability through stochastic gravitational wave signals. The G2HDM introduces a dark replica of the Standard Model electroweak gauge group, inducing an accidental $Z_2$ symmetry which not only leads to a simple scalar potential at tree-level but also offers a compelling vectorial dark matter candidate. Using the high temperature expansion in the effective potential that manifests gauge invariance, we find a possible two-step phase transition pattern in the model with a strong first-order transition occurring in the second step at the electroweak scale temperature. Collider data from the LHC plays a crucial role in constraining the parameter space conducive to this two-step transition. Furthermore, satisfying the nucleation condition necessitates the masses of scalar bosons in the hidden sector to align with the electroweak scale, potentially probed by future collider detectors. The stochastic gravitational wave energy spectrum associated with the phase transition is computed. The results indicate that forthcoming detectors such as BBO, LISA, DECIGO, TianQin and Taiji could potentially detect the gravitational wave signals generated by the first-order phase transition. Additionally, we find that the parameter space probed by gravitational waves can also be searched for in future dark matter direct detection experiments, in particular those designed for dark matter masses in the sub-GeV range using the superfluid Helium target detectors.