A D-brane inspired U(3)_CxU(3)_LxU(3)_R model
  Motivated by D-brane scenarios, we consider a non-supersymmetric model based on the gauge symmetry U(3)_CxU(3)_LxU(3)_R$ which is equivalent to the SU(3)^3 ``trinification'' model supplemented by three U(1)s. Two U(1) combinations are anomalous while the third U(1)_Z' is anomaly free and contributes to the hypercharge generator. This hypercharge embedding correspods to sin^2\theta_W=6/19 in the case of full gauge coupling unification. The U(3)^3 symmetry is broken down to the Standard Model by vev's of two (1,3,\bar3)- scalar multiplets supplemented by two Higgs fields in (1,3,1) and (1,1,3) representations. The latter break U(1)_Z' and provide heavy masses to the extra lepton doublets. Fermions belong to (3,\bar 3,1)+(\bar 3,1,3)+ (1,3,\bar 3) representations as in the trinification model. The model predicts a natural quark-lepton hierarchy, since quark masses are obtained from tree-level couplings, while charged leptons receive masses from fourth order Yukawa terms, as a consequence of the extra abelian symmetries. Light Majorana neutrino masses are obtained through a see-saw type mechanism operative at the SU(3)_R breaking scale of the order M_R\ge 10^9 GeV. 
G.K. Leontaris and J. Rizos (Univ. of Ioannina)