On the significance of the electroweak precision data

We elaborate on a recently suggested effective Lagrangian for charged-current and neutral-current electroweak interactions which in comparison with the standard electroweak theory contains three free parameters Δx, Δy, ɛ which quantify different sources for violations of SU(2) symmetry. Within the standard SU(2) _I × U(1) _Y electroweak theory, we present both exact and very much refined approximate analytical one-loop expressions for these parameters in terms of the canonical input, G_μ, M_Z, α( M_Z²), the top-quark mass, m_t, and the Higgs-boson mass, M_H. We re-emphasize the importance of discriminating between the empirically well-known purely fermionic (vacuum polarization) contributions to Δx, Δy, ɛ and the empirically unknown bosonic ones with respect to present and future electroweak precision tests. The parameters Δx and ɛ are hardly affected by standard bosonic corrections, while the full one-loop results for Δy differ appreciably from the ones obtained by taking into account fermion loops only. A detailed comparison with the experimental data on M _W^±/M _Z, s _W^-, Г _ℓ shows that these data start to become accurate enough to be sensitive to standard (bosonic) contributions to Δy beyond fermion loops.