Models of fermion mass matrices based on a flavor- and generation-dependent U(1) gauge symmetry

We study models of fermion mass matrices based on a flavor- and generation-dependent string-motivated U(1)_A gauge symmetry and report two new classes of solutions to the requisite consistency conditions. In particular, we explore the idea that the fundamental reason underlying the striking feature m_b, m_r ≪ m_t is that all of the elements of the down-quark and charged lepton effective Yukawa matrices actually arise from higher-dimension operators, suppressed by inverse powers of the Planck mass. We construct an explicit model embodying this idea, in which only the jk = 33 element of the Q = 2 3 effective Yukawa matrix Y_jku arises from dimension-4 operators.