HARD PHOTONS IN W PAIR PRODUCTION AT LEP-2

The properties of hard photon radiation in W pair production at LEP 2 are studied, with emphasis on the energy loss relevant to the W mass m easurement. We use a combination of the exact one-photon matrix elemen t and leading logarithmic structure functions. Defining unobservable, observable and initial-state photons in the phase space, it is shown t hat neither the one-photon matrix element nor the leading logarithmic structure functions alone give an adequate description of the energy l oss due to observable or initial-state photons. An event generator bas ed on these calculations is available.