WHICH BOSONIC LOOP CORRECTIONS ARE TESTED IN ELECTROWEAK PRECISION-MEASUREMENTS

The nature of the electroweak bosonic loop corrections to which curren t precision experiments are sensitive is explored. The set of effectiv e parameters Delta x, Delta y, and epsilon, which quantify SU(2) viola tion in an effective Lagrangian, is shown to be particularly useful fo r this purpose. The standard bosonic corrections are sizable only in t he parameter Delta y, while Delta x and epsilon are sufficiently well approximated by the pure fermion-loop prediction. By analyzing the con tributions to Delta y it is shown that the bosonic loop corrections re solved by the present precision data are induced by the change in ener gy scale between the low-energy process muon decay and the energy scal e 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 c onstant G(mu), no further bosonic loop corrections are necessary for c ompatibility between theory and experiment.