BOUNDS ON ORDINARY EXOTIC FERMION MIXING FROM LEP-1

We show that recent measurements of the partial widths of the Z boson at LEP significantly improve previous existing bounds on the mixing between ordinary fermions and possible heavy fermions with exotic SU (2) × U (1) assignments, especially for the μ and τ leptons and for the b quark. We show that in some extensions of the standard model with an additional U (1) factor (as in E6 models that we analyse in some detail), the effects of a ZZ′ mixing can produce similar effects to those due to fermion mixing and then both should be taken into account. We constrain s2f, the square of the mixing between ordinary and exotic fermions, to s2τ<0.025, s2b<0.077 and s2μ<0.024, improving the previous bounds by almost a factor 10 for the τ lepton, by a factor 5 for the b quark and by a factor 2 for the μ. © 1990.