Thermal Dark Matter Through the Dirac Neutrino Portal
  We study a simple model of thermal dark matter annihilating to standard model
neutrinos via the neutrino portal. A (pseudo-)Dirac sterile neutrino serves as
a mediator between the visible and the dark sectors, while an approximate
lepton number symmetry allows for a large neutrino Yukawa coupling and, in
turn, efficient dark matter annihilation. The dark sector consists of two
particles, a Dirac fermion and complex scalar, charged under a symmetry that
ensures the stability of the dark matter. A generic prediction of the model is
a sterile neutrino with a large active-sterile mixing angle that decays
primarily invisibly. We derive existing constraints and future projections from
direct detection experiments, colliders, rare meson and tau decays, electroweak
precision tests, and small scale structure observations. Along with these
phenomenological tests, we investigate the consequences of perturbativity and
scalar mass fine tuning on the model parameter space. A simple, conservative
scheme to confront the various tests with the thermal relic target is outlined,
and we demonstrate that much of the cosmologically-motivated parameter space is
already constrained. We also identify new probes of this scenario such as
multi-body kaon decays and Drell-Yan production of $W$ bosons at the LHC.
