Neutrino Mass , Mixing and Oscillations

Neutrinos have been playing an important role in the evolution of our understanding of elementary particles and could have some signi cant impact on the evolution of the universe. We have strong motivations to search for non-zero neutrino mass both from theoretical and experimental aspects. Various observational data which are indicating the existence of neutrino mass and avor mixing are piling up. In fact neutrino physics and astrophysics have become very active eld now. In the last decade, many new neutrino experiments have been performed and various important results were obtained. Neutrinos are regarded to have spin 1/2, charge zero, helicity -1 and known to exist in three avors, i.e., electron, muon and tau although the direct detection of tau neutrinos is still in progress [1]. Fourth neutrino, if exist, must be sterile (or electroweak singlet) in order to be consistent with the observed Z decay width at LEP experiment [2]. The combined LEP results give the number of light (not heavier than the half of Z boson mass) neutrino as,