LOOKING FOR Z′ BOSONS IN SUPERSYMMETRIC E6 Models THROUGH ELECTROWEAK PRECISION DATA

We review constraints on additional Z′ bosons predicted in supersymmetric (SUSY) E6 models from electroweak experiments — Z-pole experiments, mW measurements and the low-energy neutral current (LENC) experiments. Four representative models — χ, ψ, η and ν — are studied in some detail. We find that the improved data of parity violation in cesium atom, which is 2.2-σ away from the standard model (SM) prediction, could be explained by the exchange of the heavy mass eigenstate Z2 in the intermediate state. The improvement over the SM can be found in χ, η, ν models, where the total χ2 of the fit to the 26 data points decreases by about five units, owing to the better fit to the atomic parity violation. Impacts of the kinetic mixing between the U (1)Y and U (1)′ gauge bosons on the χ2-analysis are studied. We find that the Z′ model with (βE,δ)=(-π/4,0.2), where βE is the mixing angle between Zχ and Zψ bosons and δ denotes the kinetic mixing, shows the most excellent fit to the data: The total χ2 decreases by about seven units as compared to the SM. We introduce the effective mixing parameter ζ, a combination of the mass and the kinetic mixing parameters. The 95% CL lower mass bound of Z2 can be shown as a function of ζ. A theoretical prediction on ζ and the U (1)′ gauge coupling gE is studied for the χ, ψ, η and ν models by assuming the minimal particle content of the SUSY E6 models.