Electroweak precision data and a heavy Z′

We consider the physics of an extra U(1) gauge boson Z′, which can mix with Z through intermediate fermion loops. The loop contribution due to the heavy top quark significantly affects the low-energy observables, and for mZ′ > mZ, one can always adjust the shifts in these observables to be in the right direction suggested by experiments, when we impose the anomaly cancellation conditions for Z′.