Looking for Z' bosons in Supersymmetric E_6 Models through Electroweak   Precision Data
  We review constraints on additional Z' bosons predicted in supersymmetric (SUSY) E_6 models from electroweak experiments - Z-pole experiments, mW measurements and the low-energy neutral current (LENC) experiments. Four representative models - \chi,\psi, \eta, \nu models - are studied in some detail. We find that the improved data of parity violation in cesium atom, which is 2.2-\sigma away from the Standard Model (SM) prediction, could be explained by the exchange of the heavy mass eigenstate Z_2 in the intermediate state. The improvement over the SM can be found in \chi, \eta, \nu models, where the total \chi^2 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 \chi^2-analysis are studied. We find that the Z' model with (\beta_E, \delta)=(-\pi/4,0.2), where \beta_E is the mixing angle between Z_\chi and Z_\psi bosons and \delta denotes the kinetic mixing, shows the most excellent fit to the data: the total \chi^2 decreases by about seven units as compared to the SM. We introduce the effective mixing parameter \zeta, a combination of the mass and the kinetic mixing parameters. The 95% CL lower mass bound of Z_2 can be shown as a function of \zeta. A theoretical prediction on \zeta and the U(1)'gauge coupling g_E is studied for the \chi,\psi,\eta and \nu models by assuming the minimal particle content of the SUSY E_6 models. 
Gi-Chol Cho (Scuola Normale Superiore, Pisa)