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 mb, mr ⪡ mt 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 = 23 effective Yukawa matrix Yjku arises from dimension-4 operators.