Processes 301 and 306 represent the production of a Z boson (or virtual photon for process 301) in association with two photons. The Z∕γ* subsequently decays into either an e+e- pair (nproc=301) or neutrinos (nproc=306). Since these processes include real photons, the cross section diverges when either of the photons is very soft or in the direction of the beam. Thus in order to produce sensible results, the input file must supply values for both gammptmin and gammrapmax. Moreover, when the parameters zerowidth and removebr are set to .false. the decay Z → e-e+ (nproc=301) will include photon radiation from both leptons, so that a non-zero R(γ,ℓ)min (Rgalmin) should also be supplied. This will ensure that the cross section is well-defined. Anomalous couplings are not currently implemented for these processes.
nplotter_zgamgam.f is the default plotting routine.