Abstract
Motivated by D-brane scenarios, we consider a non-supersymmetric model based on the gauge symmetry U(3)C×U(3)L×U(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 corresponds to sin2θW=619 in the case of full gauge coupling unification. The U(3)3 symmetry is broken down to the Standard Model by vevs of two (1,3,3¯)-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 (3,3¯,1)+(3¯,1,3)+(1,3,3¯) 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 MR⩾109 GeV.
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