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 measurement. We use a combination of the exact 1-photon matrix element and leading logarithmic structure functions. Defining unobservable, observable and initial state photons in the phase space, it is shown that neither the one-photon matrix element nor the leading logarithmic structure functions alone give an adequate description of the energy loss due to observable or initial state photons. An event generator based on these calculations is available.