CONSTRAINTS ON ADDITIONAL-Z BOSONS

Z-pole, W-mass, and weak neutral-current (WNC) data as well as direct collider limits are used to constrain the mass and mixing of possible heavy Z2 bosons with couplings expected in grand unified theories and SU(2)L x SU(2)R x U(1) models. The data now enable the top-quark mass and Z2 properties to be limited simultaneously. The indirect (Z-pole, M(W), WNC) data stringently constrain the Z1(0)-Z2(0) mixing angle theta (\theta\ < 0.01 in most cases). For models with arbitrary Higgs representations, the limits on the Z2 mass M2 from indirect and direct constraints are rather weak (typically 160-400 GeV), but in specific models in which M2 and theta are correlated, the constraints are much stronger (M2 > 500-1000 GeV). The weak angle in the modified minimal subtraction scheme is well determined even allowing for extra Z's and an arbitrary Higgs structure: sin(2)theta(W)(M(Z)) = 0.2334(-0.0011)+0.0008, for the models considered, where the uncertainties include m(t), compared to the SU(2) x U(1) model value 0.2333 +/- 0.0008. The 95%-C.L. upper limits on m(t) (m(t) < 182 GeV for Higgs doublets and singlets only, and m(t) < 310 GeV for arbitrary Higgs representations) continue to hold in the presence of the extra Z's considered. The implications of these results for ordinary and supersymmetric grand unification, atomic parity violation, charged-current universality, R(e+e- --> mu+mu-, hadrons) below the Z pole, and superstring theories are discussed. A nongauge model in which the Z2 has the same couplings as the ordinary Z is included for comparison.