Flavor symmetry and split seesaw mechanism

We discuss an A4 flavor symmetry model in the context of the split seesaw mechanism. The model leads to one keV sterile neutrino as the warm dark matter and two heavy right-handed neutrinos being responsible for leptogenesis to explain the observed baryon asymmetry of the Universe. The A4 flavor symmetry predicts nearly tri-bimaximal mixing in the lepton sector. Combining the split seesaw mechanism with the flavor symmetry leads to an interesting correlation between the neutrino oscillation data and the cosmological constraint from X-ray observation.