Electroweak Precision Constraints on Vector-like Fermions

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We calculate the oblique electroweak corrections and confront with the experiments in an extension of the Standard Model. The new fields added are a vector-like weak doublet and a singlet fermion. After electroweak symmetry breaking there is a mixing between the components of the new fields, but no mixing allowed with the standard fermions. Four electroweak parameters, S, T, W, Y are presented in the formalism of Barbieri et al., these are the generalization of the Peskin-Takeuchi S, T, U's. The vector-like extension is slightly constrained, T requires the new neutral fermion masses not to be very far from each other, allowing higher mass difference for higher masses and smaller mixing. S, W, Y gives practically no constraints on the masses. This extension can give a positive contribution to T, allowing a heavy Higgs boson in electroweak precision tests of the Standard Model.