Dark photon portal to dark matter, long-lived particles and gravitational waves

Dark photons arising at or below the GeV scale from a spontaneously broken gauge symmetry are one of the simplest and most exciting extensions of the Standard Model. They couple to known particles via kinetic mixing with the Standard Model gauge bosons and can act as mediators between visible matter and new hidden states, such as sub-GeV dark matter particles. I will show that these models can satisfy all current constraints from laboratory experiments, astrophysical observations and cosmological data and predict new signals that can be targeted with future accelerator experiments. Moreover, if the dark sector features a classically conformal symmetry, spontaneous symmetry breaking can happen via a strong first-order phase transition. For MeV-scale dark sectors, the resulting gravitational wave signals are predicted to lie in the nano-Hertz frequency range and can fit the stochastic background observed by pulsar timing arrays. In this case it becomes crucial to understand the role that the dark Higgs boson plays in the early universe and in the evolution of the dark matter abundance.