Universal Seesaw Mass Matrix Model and Neutrino Phenomenology

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 $m_f$ by the conventional seesaw mechanism M_f\simeq m_L M_F^{-1}m_R (M_F is a mass matrix of hypothetical heavy fermions F). On the other hand, the observed fact m_t\sim \Lambda_L=O(m_L) (electroweak scale \Lambda_L=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 det M_F=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 \Lambda_L without the sesaw-suppression factor O(m_R)/O(M_F). For neutrino sector, the mass matrix M_\nu is given by M_\nu \simeq m_L M_F^{-1} m_L^T (F=N), so that the masses m_\nu are suppressed by a factor O(m_L)/O(m_R) compared with the conventional quark and charged lepton masses. The model can naturally lead to a large mixing \sin^2 2\theta \simeq 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 \nu_{Li} and SU(2)_L\timesSU(2)_R singlet neutrinos N_{1i} (i=e,\mu,\tau), so that the model can give a simultaneous explanation of the atmospheric and solar neutrino data.