Large Lepton Flavor Mixing and E_6-type Unification Models

There are experimental indications of large flavor mixing between $\nu_{\mu }$ and $\nu_{\tau}$. In the unification models, in which the massless sector includes extra particles beyond the standard model, there possibly appear the mixings between quarks (leptons) and the extra particles. When large mixings occur, lepton flavor mixings can be quite different from quark flavor mixings. By taking the string inspired $SU(6) \times SU(2)_R$ model with global flavor symmetries, we obtain the neutrino flavor mixing $\sin \theta_{23} \simeq \lambda = \sin \theta_C$ around the unification scale. It can be expected that due to large Yukawa couplings of neutrinos, the renormalization effect increases $\sin 2 \theta_{23}$ naturally up to $\sim 1$ at the electroweak scale. Fermion mass spectra and the CKM matrix of quarks obtained in this paper are also phenomenologically viable.