Z′ reservation at LEP2

We consider the possiblity that one extra Z ≡ Z′ exists with arbitrary mass and fermion couplings that do not violate (charged) lepton universality. We show that, in such a situation, a functional relationship is generated between the deviations from the SM values of three leptonic observables of two-fermion production at future e+e− colliders that is completely independent of the values of the Z′ mass and couplings. This selects a certain region in the 3-d space of the deviations that is characteristic of the model (Z′ “reservation”). As a specific and relevant example, we show the picture that would emerge at LEP2 under realistic experimental conditions.