Probing Z - Z' mixing at future e+e- colliders

We discuss the possibility of obtaining constrains on Z - Z' mixing from studies of the process e+e- → W+W- at planned high energy e+e- colliders, and derive the corresponding limits on the Z' mass for different extended models. Our results indicate that the limits on the mixing angle are quite stringent, and can typically reach the level of 10-5 - 10-6. We also present a detailed comparison with the potential of the reaction e+e- → ff-bar which shows that significantly better sensitivity to Z - Z' mixing should be expected from W+W- production. (author). 22 refs, 14 figs, 3 tabs