Z' interference effects from TRISTAN to LEP2

We discuss the potential of present e^+e^- colliders to probe for a heavy Z^' boson via its interference effects, in a model-independent framework. The leptonic channel is unambiguous, and thus favourable over the hadronic ones. The effect on the leptonic cross section $\sigma_{\mu\mu}$, induced by a heavy Z^' with arbitrary couplings, is uniquely determined at LEP2 energies. It results in a negative deviation from the standard model prediction, due to destructive $\gamma-Z'$ and Z-Z' interferences. At TRISTAN energies the former dominates and is given by the vector coupling of the Z^'. Around the Z peak, the Z-Z' interference dominates and depends on the axial-vector coupling. The forward-backward asymmetry is complementary to $\sigma_{\mu\mu}$ in the Z^' search. Comparison of the e^+e^- potentials indicates that already after one year of operation, LEP2 will achieve a sensitivity to Z^' that exceeds the current one at TRISTAN.