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 comparis on with the standard electroweak theory contains three free parameters Delta x, Delta y, epsilon which quantify different sources for violat ions of SU(2) symmetry. Within the standard SU(2)(I) x U(1)(Y) electro weak theory, we present both exact and very much refined approximate a nalytical one-loop expressions for these parameters in terms of the ca nonical input, G mu, M(Z), alpha(M(Z)(2)), the top-quark mass, m(t), a nd the Higgs-boson mass, M(H). We re-emphasize the importance of discr iminating between the empirically well-known purely fermionic (vacuum polarization) contributions to Delta x, Delta y, epsilon and the empir ically unknown bosonic ones with respect to present and future electro weak precision tests. The parameters Delta x and epsilon are hardly af fected by standard bosonic corrections, while the full one-loop result s for Delta y differ appreciably from the ones obtained by taking into account fermion loops only. A detailed comparison with the experiment al data on M(W)+/-/M(Z), S-W(-2), Gamma e shows that these data start to become accurate enough to be sensitive to standard (bosonic) contri butions to Delta y beyond fermion loops.