Universal Seesaw Mass Matrix Model

Stimulated by the recent development of the “universal seesaw mass matrix model”, an application of the model to the neutrino mass matrix is investigated: For the charged lepton and down-quark sectors, the model explains the smallness of their masses mf by the conventional seesaw mechanism Mf ≃ mLM F mR (MF is a mass matrix of hypothetical heavy fermions F ). On the other hand, the observed fact mt ∼ ΛL = O(mL) (electroweak scale ΛL = 174 GeV) seems to reject the applying of the seesaw mechanism to the up-quark sector. However, recently, it has been found that, by taking detMF = 0 for the up-quark sector F = U , we can understand the question of why only top quark has a mass of the order of ΛL without the sesaw-suppression factor O(mR)/O(MF ). For neutrino sector, the mass matrix Mν is given by Mν ≃ mLM F mL (F = N), so that the masses mν are suppressed by a factor O(mL)/O(mR) compared with the conventional quark and charged lepton masses. The model can naturally lead to a large mixing sin 2θ ≃ 1. Also another model is investigated within the framework of the universal seesaw model: the model leads to three sets of the almost degenerate two Majorana neutrinos which are large mixing states between the left-handed neutrinos νLi and SU(2)L×SU(2)R singlet neutrinos N1i (i = e, μ, τ), so that the model can give a simultaneous explanation of the atmospheric and solar neutrino data.