Gravitational Waves from Dark Gauge Sectors

We explore gravitational-wave (GW) signatures from a strong first-order phase transition in a non-Abelian dark sector, which naturally gives rise to vector dark matter. We consider a general class of models featuring a new dark gauge sector communicating with the Standard Model (SM) through a Higgs portal and a vector-like fermionic portal. We also study the scenario where the dark sector interacts with the SM only via gravity. In all cases, we scan the full parameter space and analyse GW production, highlighting that secluded dark sectors, with excessive dark radiation, fail to produce a LISA-observable GW background. Notably, the fermionic portal yields distinctive GW signals at LISA with peak frequencies of 1-10 mHz, reaching up to 1 Hz for future interferometers like BBO and DECIGO, while the Higgs portal scenario remains limited to around 1 mHz. Both frameworks account for the observed dark matter abundance and predict detectable LISA signals for dark vector bosons near 3-4 TeV (Higgs portal) and 10 TeV (fermionic portal), with a $\sim$10 GeV dark Higgs. Finally, we identify a unique six-top final state from pair-produced vector-like fermions, offering a striking collider signature within HL-LHC reach. Its detection would provide a smoking-gun signal for the fermionic portal, establishing complementarity between collider, GW, and dark matter signals.