Orateur
Description
We investigate a flavoured horizontal gauge extension of the Standard Model (SM), and demonstrate that first-order phase transitions (FOPTs) generically arise during its TeV-scale symmetry breaking. These FOPTs occur only for flavour gauge couplings $g_f \sim \mathcal{O}(1); beyond this, one-loop corrections to the quartic interaction start to dominate the thermal potential and weaken the transition. To obtain reliable predictions for the resultant gravitational-wave (GW) signal, we match each phase transition to its steady-state hydrodynamical solution, capturing out-of-equilibrium plasma effects through an effective friction parameter primarily sourced by soft flavour gauge boson interactions with the SM plasma. Within this framework, we identify both runaway solutions for sufficiently strong FOPTs and regimes in which the interplay between hydrodynamics and friction leads to enhanced GW production despite a reduction in the transition strength. We find that the resulting stochastic GW signals can lie within the sensitivity band of next-generation detectors such as the Einstein Telescope.