Radiative Corrections to W-pair Mediated Four-fermion Production at Lep2

as typical example, but all four-fermion final states originating from W -boson pairs can be treated in a similar way. Since the RADCOR 98 conference took place our results have been finalized and discussed elsewhere . A highlight of the main points is given here. For related issues, see these proceedings . A complete calculation of O(α) radiative corrections to a process with six external particles is beyond present possibilities. However, the possible presence of two resonances as intermediate state in reaction (1) offers an additional expansion parameter ΓW /MW besides α. Since the ratio between the width and mass of the W boson is also of order α, a double expansion both in α and ΓW /MW is a natural and economic way to simplify the calculation. Thus the aim is to calculate radiative corrections of O(α) and O(ΓW /MW ), while neglecting higher-order terms such as αΓW /MW . The way in which the expansion is carried out is called the double-pole approximation 3 (DPA), which possesses the important feature of maintaining gauge invariance.