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

TL;DR

This paper investigates how nonuniversal quark family assignments in 3-4-1 models affect neutral current interactions, using experimental data to constrain the mixing angles and mass scales of new gauge bosons.

Contribution

It analyzes the impact of quark family nonuniversality in 3-4-1 models on experimental bounds and explores different family assignment scenarios without exotic electric charges.

Findings

Heaviest quark family must transform differently to satisfy experimental bounds.

Constraints on mixing angles between neutral currents are derived.

Lower bounds on new gauge boson masses are established.

Abstract

Flavour changing neutral currents arise in the $SU(3)_c\otimes SU(4)_L\otimes U(1)_X$ extension of the standard model because anomaly cancellation among the fermion families requires one generation of quarks to transform differently from the other two under the gauge group. In the weak basis the distinction between quark families is meaningless. However, in the mass eigenstates basis, the Cabibbo-Kobayashi-Maskawa mixing matrix motivates us to classify left-handed quarks in families. In this sense there are, in principle, three different assignments of quark weak eigenstates into mass eigenstates. In this work, by using measurements at the Z-pole, atomic parity violation data and experimental input from neutral meson mixing, we examine two different models without exotic electric charges based on the 3-4-1 symmetry, and address the effects of quark family nonuniversality on the bounds on the mixing angle between two of the neutral currents present in the models and on the mass scales $M_{Z_2}$ and $M_{Z_3}$ of the new neutral gauge bosons predicted by the theory. The heaviest family of quarks must transform differently in order to keep lower bounds on $M_{Z_2}$ and $M_{Z_3}$ as low as possible without violating experimental constraints.