Sterile Neutrinos for Warm Dark Matter and the Reactor Anomaly in Flavor
  Symmetry Models
  We construct a flavor symmetry model based on the tetrahedral group A_4 in
which the right-handed neutrinos from the seesaw mechanism can be both keV warm
dark matter particles and eV-scale sterile neutrinos. This is achieved by
giving the right-handed neutrinos appropriate charges under the same
Froggatt-Nielsen symmetry responsible for the hierarchy of the charged lepton
masses. We discuss the effect of next-to-leading order corrections to deviate
the zeroth order tri-bimaximal mixing. Those corrections have two sources: (i)
higher order seesaw terms, which are important when the seesaw particles are
eV-scale, and (ii) higher-dimensional effective operators suppressed by
additional powers of the cut-off scale of the theory. Whereas the mixing angles
of the active neutrinos typically receive corrections of the same order, the
mixing of the sterile neutrinos with the active ones is rather stable as it is
connected with a hierarchy of mass scales. We also modify an effective A_4
model to incorporate keV-scale sterile neutrinos.
