Non-zero \theta_13 in models for hierarchical neutrino mass spectrum
  We introduce three right-handed Majorana neutrinos and combine the type-I seesaw and inert doublet mechanisms. The resultant (active) neutrino mass matrix is divided into rank=1 and =2 parts with different energy scales. The different energy scales are reduce to different mass scales in the hierarchical neutrino mass spectrum. We apply this scheme to both the inverted and normal hierarchy cases and find a correlation between the smallest mixing angle (\theta_13) and the lightest neutrino mass. 
Takeshi Araki
hep-ph