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

We report the most general expression for the chiral charges of aZ′gauge boson comingfrom anE6unification model, as a function of the electroweak parameters and thecharges of theU(1) factors in the chain of subgroups. These charges are valid for anarbitrary Higgs sector and only depend on the branching rules of theE6fundamentalrepresentation and the corresponding rules for the fermionic representations of theirsubgroups. By assumingE6unification, the renormalization group equations (RGE)allow us to calculate the electroweak parameters at low energies for most of the chains ofsubgroups inE6. From RGE and unitary conditions, we show that at low energies theremust be a mixing between the gauge boson of the standard model hypercharge and theZ′. From this, it is possible to delimit the preferred region in the parameter space fora breaking pattern inE6. In general, without unification, it is not viable to determinethis region; however, for some models and under certain assumptions, it is possible tolimit the corresponding parameter space. By using the most recent upper limits on thecross-section of extra gauge vector bosonsZ′decaying into dileptons from the ATLASdata at 13 TeV with accumulated luminosities of 36.1 fb−1and 13.3 fb−1, we report the95% C.L. lower limits on theZ′mass for the typicalE6benchmark models. We alsoshow the contours in the 95% C.L. of theZ′mass bounds for the entire parameter space of E6