Bounds on ordinary-exotic fermion mixing from LEP-1

Abstract

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 E 6 models that we analyse in some detail), the effects of a ZZ′ mixing can produce similar effects to those due to fermion mixing and then both should be taken into account. We constrain s 2 f , the square of the mixing between ordinary and exotic fermions, to s 2 τ <0.025, s 2 b<0.077 and s 2 μ <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.