Exploring portals to a hidden sector through fixed targets

We discuss the sensitivity of neutrino experiments at the luminosity frontier to generic hidden sectors containing new (sub)-GeV neutral states. The weak interaction of these states with the standard model can be efficiently probed through all of the allowed renormalizable "portals" (in the Higgs, vector, and neutrino sectors) at fixed target proton beam facilities, with complementary sensitivity to colliders. We concentrate on the kinetic-mixing vector portal, and show that certain regions of the parameter space for a new U(1)(S) gauge sector with long-lived sub-GeV mass states decaying to standard model leptons are already severely constrained by the data sets at LSND, MiniBooNE, and NuMI/MINOS. Furthermore, scenarios in which portals allow access to stable neutral particles, such as MeV-scale dark matter, generally predict that the neutrino beam is accompanied by a "dark matter beam," observable through neutral-current-like interactions in the detector. As a consequence, we show that the LSND electron recoil event sample currently provides the most stringent direct constraint on MeV-scale dark matter models.