Constraints on extended neutral gauge structures

Indirect precision data are used to constrain the masses of possible extra Z′ bosons and their mixings with the ordinary Z. We study a variety of Z′ bosons as they appear in E₆ and left-right unification models, the sequential Z boson, and the example of an additional U(1) in a concrete model from heterotic string theory. In all cases the mixings are severely constrained (sin θ < 0.01). The lower mass limits are generally of the order of several hundred GeV and competitive with collider bounds. The exception is the Z_ψ boson, whose vector couplings vanish and whose limits are weaker. The results change little when the p parameter is allowed, which corresponds to a completely arbitrary Higgs sector. On the other hand, in specific models with minimal Higgs structures the limits are generally pushed into the TeV region.