Phase transitions and gravitational waves in a non-abelian vector dark matter scenario

TL;DR

The paper investigates a non-Abelian dark sector based on SU(2)_D where custodial symmetry stabilizes dark vector bosons as dark matter. It combines theoretical consistency, collider, and cosmological constraints to identify viable regions in the parameter space (g_D, m_V_D, m_H_D, θ_S) and then analyzes the finite-temperature scalar potential to reveal strong first-order phase transitions, computing the resulting gravitational-wave spectra. The study shows that a subset of parameter space yields the correct dark-matter relic density while producing gravitational waves with peak amplitudes and frequencies within the reach of future space-based detectors such as LISA, DECIGO, BBO, TianQin, and Taiji, with the strongest signals typically arising from the φ → φ_D transition. This work demonstrates that gravitational-wave observations can provide a complementary and powerful probe for beyond-Standard-Model scenarios involving vector dark matter and cosmological phase transitions, effectively narrowing the viable model parameter space.

Abstract

We study a scenario where the Standard Model is extended by a SU(2) gauge group in the dark sector. The three associated dark gauge bosons are stabilised via a custodial symmetry triggered by an additional dark SU(2) scalar doublet, thus making them viable dark-matter candidates. After considering the most recent constraints for this model, we analyse the phase transition dynamics and compute the power spectrum of resulting stochastic gravitational-wave background. Finally, we find regions of the parameter space yielding the observed dark-matter relic density while also leading to strong enough phase transition with an associated gravitational-wave signal reaching the sensitivity of future space-based gravitational-wave detector, such as LISA, DECIGO, BBO, TianQin or Taiji.

Figures (11)

• Figure 1: Annihilation processes, involving SM particles or $H_D$ in the final states (top row) or $V_D$ and scalar particles in the final state (bottom row). $V_\text{SM}$ and $f_\text{SM}$ respectively stand for massive vector and fermion SM particles.
• Figure 2: Areas in the $\{m_{V_D},m_{H_D}\}$ projection of the parameter space which are allowed by astrophysical and cosmological constraints: underabundant and exact relic density (top left), underabundant and allowed by direct detection experiments (top right), underabundant and allowed by indirect detection from Fermi-LAT (bottom left) and underabundant and allowed by neutrino fluxes measured at IceCube (bottom right).
• Figure 3: Combination of all astrophysical and cosmological constraints in the $\{m_{V_D},m_{H_D}\}$ projection.
• Figure 4: Allowed areas in the $\{m_{V_D},m_{H_D}\}$ projection of the $SU(2)_D$ model parameter space allowed by theoretical and collider constraints. From top left to bottom right: perturbative unitarity, EW precision tests, HiggsBounds and HiggsSignals.
• Figure 5: Combination of all collider and theoretical constraints in the $\{m_{V_D},m_{H_D}\}$ (left) and $\{m_{H_D},\cos\theta_S\}$ (right) projections.