Fermion Masses and Mixings in String Inspired SU(6)×SU(2)_R Model

抄録

We showed that the characteristic patterns of fermion masses and mixings can be under-stood in the framework of string inspired SU(6)×SU(2)_R model. This model contains two key ingredients. The first is the flavor symmetry which generates mass hierarchies via Froggatt-Nielsen mechanism. The second is the appearance of large D^c-g^c and L-H_d mixings. The mass hierarchy of up-type quarks is a direct result of the flavor symmetry. On the other hand, for down-type quarks there appears a mixing between D^c and g^c, which are both SU(2)_L-singlets. Due to large D^c-g^c mixing mass hierarchy of down-type quarks differs from that of up-type quarks. For leptons a mixing between SU(2)_L-doublets L and H_d occurs. In the neutrino sector Dirac mass hierarchies cancel out in large part due to seesaw mechanism. Thus we obtain the mass hierarchies m_u:m_c:m_t 〜 λ^7:λ^4:1, (93) m_d:m_s:m_b 〜λ^7:λ^6:λ^3, (94) m_e:m_μ:m_τ 〜 λ^7:λ^<4.5>:λ^2, (95) m_<ν1>:m_<ν2>:m_<ν3> 〜 λ^6:λ^5:λ^4. (96) Nontrivial V_<CKM> is induced by large D^c-g^c mixings. In the present model V_<ub> is naturally suppressed compared to V_<td>. We understood the reason why the V_<CKM> is nearly equal to unity but not exactly unity. Further we obtain a phenomenologically viable relation V_<td> = V_<cd>・V_<ts>. For V_<MNS> the present model can yield a large mixing angle solution with tanθ_<12>, tanθ_<23> = Ο(√λ).