The simplest 3-3-1 model

A simple extension of the Standard Model (SM), based on the gauge group SU(3)C ⊗SU(3)L ⊗U(1)Y with Y being the hypercharge, is considered. We show that, by imposing an approximate global SU(2)L ×SU(2)R custodial symmetry at the SM energy scale, the Z − Z′ mixing is absent at tree level and the value of the ρ parameter can be kept close to one. Tree-level flavor-changing neutral currents (FCNCs) are also reduced to three particles, namely Z′, a CP-odd Higgs and a CP-even Higgs. The model predicts new leptons with electric charges of ±1/2e and new quarks with ±1/6e charges as well as new gauge and scalar bosons with ±1/2e charges. Electric charge conservation requires that one of them must be stable. Their masses are unfortunately free parameters.