ON THE COULOMB EFFECTS ON THE W LINE-SHAPE AT LEP2

One of the most powerful methods for obtaining a precision measurement of M(w) at LEP2 is from direct reconstruction of the invariant mass d istribution of W bosons produced in e(+)e(-) --> W+W-. We investigate the effects on the W line shape, and in particular on the average inva riant mass, of phase-space and first-order Coulomb corrections. The la tter are shown to have a non-negligible effect on the reconstructed ma ss, inducing shifts of order -20 MeV at collision energies above thres hold, compared to the Born approximation cross section. The sign and m agnitude of the effect can be understood in a simple model calculation in which one of the W bosons is assumed to be stable.