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Flavour changing neutral currents arise in the $SU(3)_c\otimes SU(4)_L\otimesU(1)_X$ extension of the standard model because anomaly cancellation among thefermion families requires one generation of quarks to transform differentlyfrom the other two under the gauge group. In the weak basis the distinctionbetween quark families is meaningless. However, in the mass eigenstates basis,the Cabibbo-Kobayashi-Maskawa mixing matrix motivates us to classifyleft-handed quarks in families. In this sense there are, in principle, threedifferent assignments of quark weak eigenstates into mass eigenstates. In thiswork, by using measurements at the Z-pole, atomic parity violation data andexperimental input from neutral meson mixing, we examine two different modelswithout exotic electric charges based on the 3-4-1 symmetry, and address theeffects of quark family nonuniversality on the bounds on the mixing anglebetween two of the neutral currents present in the models and on the massscales $M_{Z_2}$ and $M_{Z_3}$ of the new neutral gauge bosons predicted by thetheory. The heaviest family of quarks must transform differently in order tokeep lower bounds on $M_{Z_2}$ and $M_{Z_3}$ as low as possible withoutviolating experimental constraints.

Effects of quark family nonuniversality in SU(3)_c X SU(4)_L X U(1)_x models