Electroweak couplings and LHC constraints on alternative Z‧ models in E6

We report the most general expression for the chiral charges of a Z‧ gauge boson coming from an E6 unification model, as a function of the electroweak parameters and the charges of the U(1) factors in the chain of subgroups. These charges are valid for an arbitrary Higgs sector and only depend on the branching rules of the E6 fundamental representation and the corresponding rules for the fermionic representations of their subgroups. By assuming E6 unification, the renormalization group equations (RGE) allow us to calculate the electroweak parameters at low energies for most of the chains of subgroups in E6. From RGE and unitary conditions, we show that at low energies there must be a mixing between the gauge boson of the standard model hypercharge and the Z‧. From this, it is possible to delimit the preferred region in the parameter space for a breaking pattern in E6. In general, without unification, it is not viable to determine this region; however, for some models and under certain assumptions, it is possible to limit the corresponding parameter space. By using the most recent upper limits on the cross-section of extra gauge vector bosons Z‧ decaying into dileptons from the ATLAS data at 13 TeV with accumulated luminosities of 36.1 fb-1 and 13.3 fb-1, we report the 95% C.L. lower limits on the Z‧ mass for the typical E6 benchmark models. We also show the contours in the 95% C.L. of the Z‧ mass bounds for the entire parameter space of E6.