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 $\Delta x, \Delta y$,$\varepsilon$whichquantifydifferentsourcesforviolationsof$SU(2)$symmetry\mathrm{.}Withinthestandard$SU(2)_I \times U(1)_Y$ electroweak theory, we present both exact and very much refined approximate analytical one-loop expressions for these parameters in terms of the canonical input, $G_{\mu },\text{MZ}$, $\text{alpz}$, the top-quark mass, $\text{Mt}$, and the Higgs-boson mass, $\text{MH}$. We reemphase the importance of discriminating between the {\it empirically well-known purely fermionic} (vacuum polarization) contributions to $\Delta x, \Delta y$,$\varepsilon$ and the {\it empirically unknown bosonic} ones with respect to present and future electroweak precision tests. The parameters $\mathrm{\Delta }x$ and $\epsilon$ are hardly affected by standard bosonic corrections, while the full one-loop results for $\mathrm{\Delta }y$ differ appreciably from the ones obtained by taking into account fermion loops only. A detailed comparison with the experimental data on $\text{MWpm}\mathrm{/}\text{MZ}$, ${\overline{\text{sw}}}^2$, ${\mathrm{\Gamma }}_l$ shows that these data start to become accurate enough to be sensitive to standard (bosonic) contributions to $\mathrm{\Delta }y$ beyond fermion loops.