New Aspects on a Seesaw Mass Matrix Model

Abstract

Recent development of the universal seesaw mass matrix model is reviewed. The model was proposed in order to explain why quark and lepton masses are so small compared with the electroweak scale \Lambda_L =<\phi_L^0>=174 GeV. However, the recently observed top-quark mass m_t\simeq 180 GeV seems to make an objection against the seesaw mass picture. For this problem, it has recently pointed out that the seesaw mass matrix model is rather favorable to the fact m_t\sim \Lambda_L if we consider the model with det M_F=0 for up-quark sector, where M_F is a 3 x 3 mass matrix of hypothetical heavy fermions F. The model can give a natural explanation why only top-quark acquire the mass of the order of \Lambda_L. The model with det M_U=0 offers abundant new physics to us (e.g., the fourth up-quark t', FCNC, and so on).