SU(4)_L X U(1)_X three-family model for the electroweak interaction
  An extension of the gauge group $SU(2)_L\otimes U(1)_Y$ of the standard model to the symmetry group $SU(4)_L\otimes U(1)_X$ (3-4-1 for short) is presented. The model does not contain exotic electric charges and anomaly cancellation is achieved with a family of quarks transforming differently from the other two, thus leading to FCNC. By introducing a discrete $Z_2$ symmetry we obtain a consistent fermion mass spectrum, and avoid unitarity violation of the CKM mixing matrix arising from the mixing of ordinary and exotic quarks. The neutral currents coupled to all neutral vector bosons are studied, and by using CERN LEP and SLAC Linear Collider data at Z-pole and atomic parity violation data, we bound parameters of the model related to tree-level $Z-Z^\prime$ mixing. These parameters are further constrained by using experimental input from neutral meson mixing in the analysis of sources of FCNC present in the model. Constraints coming from the contribution of exotic particles to the one-loop oblique electroweak parameters $S$, $T$ and $U$ are also briefly discussed. Finally, a comparison is done of the predictions of different classes of 3-4-1 models without exotic electric charges. 
Luis A. Sanchez, Luis A. Wills-Toro and Jorge I. Zuluaga