Speaker
Description
Topological defects arising from cosmological phase transitions provide a direct link between early-Universe cosmology and particle physics. The two-Higgs-doublet model (2HDM) exhibits a rich vacuum structure, admitting a variety of topological defects including domain walls, strings and monopoles. A $\mathbb{Z}_2$ symmetry is often imposed to eliminate flavour-changing-neutral-currents (FCNCs), naturally leading to the formation of domain walls upon spontaneous symmetry breaking.
We present a systematic classification of domain wall solutions in the 2HDM, in both global and gauged settings. Distinct classes of solutions are identified, characterised by local symmetry breaking within the defect core. These include configurations exhibiting electromagnetic symmetry violation through charged condensates, as well as solutions with localised CP-violating profiles, arising from different orderings of the scalar particle masses.
From a cosmological perspective, domain walls must ultimately collapse to avoid overclosing the Universe, resulting in a transient, out-of-equilibrium environment, during which exotic particle interactions could occur. The decay of such networks is expected to source a stochastic gravitational wave background, offering a potential observational window into extended scalar sectors. We discuss the implications of these signatures, and outline ongoing work aimed at quantifying the resulting gravitational wave spectra. Such observations could provide novel constraints on the parameter space of the 2HDM, linking collider scale physics to cosmological data.