Virtual effects of heavy chiral fermions at e+e- colliders

We derive the low-energy electroweak effective Lagrangian for the case of additional heavy, unmixed, sequential fermions. Present LEP1 data still allow the presence of new fermionic doublets (quarks and/or leptons), with masses greater than MZ/2, provided that these multiplets are sufficiently degenerate. Deviations of the effective Lagrangian predictions from full one-loop computation are sizeable only for fermion masses close to the threshold MZ/2. We analyse the contribution of new heavy sequential fermions to the e+e- → W+W- cross-section at the energies of LEP2 and Next Linear Collider (NLC). The signals coming from an additional doublet will be out of the LEP2 observability level. More interesting is the case of NLC where the simultaneous presence of the kinematic enhancement factor p2/m2W, and of the unitarity delay effect gives deviations from the Standard Model of the order of 10-50 percent, for a wide range of new fermion masses. (author)