Which Bosonic Loop Corrections are Tested in Electroweak Precision Measurements?

The nature of the electroweak bosonic loop corrections to which current precision experiments are sensitive is explored. The set of effective parameters $\De x$, $\De y$, and $\eps$, which quantify SU(2) violation in an effective Lagrangian, is shown to be particularly useful for this purpose. The standard bosonic corrections are sizable only in the parameter $\De y$, while $\De x$ and $\eps$ are sufficiently well approximated by the pure fermion-loop prediction. By analyzing the contributions to $\De y$ it is shown that the bosonic loop corrections resolved by the present precision data are induced by the change in energy scale between the low-energy process muon decay and the energy scale of the LEP1 observables. If the (theoretical value of the) leptonic width of the W boson is used as input parameter instead of the Fermi constant $\GF$, no further bosonic loop corrections are necessary for compatibility between theory and experiment.