Dark Neutrinos and a Three Portal Connection to the Standard Model

Abstract: We introduce a dark neutrino sector which respects a hidden $U(1)^\prime$ gauge symmetry, subsequently broken by the vacuum expectation value of a dark scalar. The model is a self-consistent realisation of an extended hidden sector that communicates with the SM only via the three renormalizable portals, namely neutrino, vector and scalar mixing. The interplay between portal couplings leads to several novel signatures in heavy neutrino, dark photon, and dark scalar searches, typically characterised by multi-leptons plus missing energy and displaced vertices. A striking signature arises in kaon factories such as NA62, where $K^+\to\ell_\alpha^+\nu \ell_\beta^+\ell_\beta^-$ decays could reveal a heavy neutrino and a light dark photon resonance above backgrounds. Given the open parameter space, we also comment on recent ideas to explain outstanding experimental anomalies, and how they would fit in our proposed model. A minimal extension of the model, possibly motivated by anomaly cancellation, can accommodate a dark matter candidate strongly connected to the neutrino sector.