left-handed (two-component) neutrinos νe, νμ, and ντ with the left-handed fields se, sμ, and sτ that are the charge conjugates of the right-handed neutrino fields νre, νrμ, νrτ , i.e., se = −iσ2 ν∗ re = (ν† re2,−ν re1), etc. The six left-handed neutrino fields form a six-vector, N = (νe, νμ, ντ , se, sμ, sτ ). The only mass term available for two left-handed neutrino fields Ni and Nj isMij (Ni1...

A seesaw mass matrix model is reviewed as a unification model of quark and lepton mass matrices. The model can understand why topquark mass mt is so singularly enhanced compared with other quark masses, especially, why mt ≫ mb in contrast to mu ∼ md, and why only top-quark mass is of the order of the electroweak scale ΛW , i.e., mt ∼ O(ΛW ). The model predicts the fourth up-quark t′ with a mass...