  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 secluded U(1)
gauge sector with long-lived sub-GeV mass states decaying to Standard Model
leptons are already severely constrained by the datasets 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.
