Mirror fermions and LEP precision data

The three generation mirror fermion model is compared with LEP precision and low energy data. While for zero mixing of ordinary and mirror fermions the model is not favoured, for non zero mixing very low χ fits have been obtained, in particular when right handed leptonic mixings are small and left handed leptonic mixings are large (of the order of 0.1 radians.) The validity of the standard model (SM) has been successfully tested at the one loop level by the LEP data [1]. The precise experimental data severly limit the possibility of any kind of new physics. It is thus compulsory to confront any hypothetical extension of the standard model with the precise LEP data. In this note we consider the possible existence of mirror fermions. The very simple extension of the SM of [2] is to enlarge only the fermion content by introducing mirror (i.e. opposite chirality property) fermions to each fermion of the SM (i.e. to each ordinary fermion), preserving the SU(2) ⊗ U(1) group structure. Ordinary and mirror fermions are allowed to mix. In fact mixing is necessary in order to avoid stable mirror fermions. Present experiments directly exclude mirror fermions with masses below roughly half of the Z mass. More massive mirror fermions are still allowed. In the following we consider heavy mirror fermions with masses exceeding the weak vector boson masses. Many of the phenomenological consequences of such a model have been worked out in [3], [4], where motivations for the model are also discussed. To compare the mirror model with LEP data one has to calculate the one loop corrections to the physical observables. This is particularly easy in the case of no mixing between ordinary and mirror fermions. Since LEP observables refer only to ordinary fermions it is easy to see that the effects