Despite its great success, we know that the Standard Model is not the ultimate theory of particles and interactions: it cannot explain neutrino masses, dark matter, our own existence (i.e. the Universe baryon asymmetry). A New Standard Theory must exist.
The key question is: what is the new physics scale? In the past decades, a formidable theoretical and experimental effort took place to explore the TeV scale, culminating in the LHC experiments. No evidence of new physics has been found so far.
A change of paradigm may be required. New physics at low energies, the so-called dark sectors, offers a viable alternative. Their theoretical study and experimental searches are blooming. Heavy neutral leptons, dark photons, dark scalars are being widely studied. From a theoretical perspective, their role in explaining neutrino masses, the baryon asymmetry of the Universe and dark matter is being investigated. From the experimental point of view, there are already very sensitive searches for these hypothetical particles and the next decade offers a unique window of opportunity reaching an unprecedented sensitivity: from the SBN programme, to ProtoDUNE, NA64, KM3Net, SHiP complemented by beam-dump, collider, sub-GeV dark matter, high energy neutrinos and gravitational wave searches.
In this Colloquium I will discuss what dark sectors are and how they can offer a solution to the unexplained evidence of physics beyond the Standard Model. I will review current searches and future opportunities, with emphasis on rich (i.e. non-minimal) dark sectors at the GeV scale. Thanks to their multiple particles and interactions, they have strong theoretical motivation, can explain the 2023 evidence of nanoHertz gravitational waves and have nearly-unexplored phenomenology.
