Neutrino models with a zero mass eigenvalue

Absolute values of the neutrino mass are not known still now although their upper bounds are constrained through several experiments and observations. Recent analyses of cosmological observations present severe constraint on the sum of neutrino masses. It might suggest an interesting possibility for the absolute values of neutrino mass and their ordering. In this paper, taking it as a useful hint, we study possible neutrino models with a zero mass eigenvalue from a view point of neutrino oscillation data and baryon number asymmetry in the Universe. We focus our study on the seesaw type mass generation by making a certain assumption for origin of right-handed neutrino mass.