Comparison of Discovery Limits for Extra Z Bosons at Future Colliders

We study and compare the discovery potential for heavy neutral gauge bosons ($Z'$) at various $e^+e^-$ and $p\stackrel{(-)}{p}$ colliders that are planned or have been proposed. Typical discovery limits are for the TEVATRON $\sim 1$ TeV, DI-TEVATRON $\sim 2$ TeV, LHC $\sim 4$ TeV, LSGNA (a 60 TeV $pp$ collider) $\sim 13$ TeV while the $e^+e^-$ discovery limits are $2-10\times \sqrt{s}$ with the large variation reflecting the model dependence of the limits. While both types of colliders have comparable discovery limits the hadron colliders are generally less dependent on the specific $Z'$ model and provide more robust limits since the signal has little background. In contrast, discovery limits for $e^+e^-$ limits are more model dependent and because they are based on indirect inferences of deviations from standard model predictions, they are more sensitive to systematic errors.