These are reduced if we can measure processes in which the emerging particles from the QCD subprocess are not themselves subject to QCD effects. 1 illustrates the different types of process. The particular virtue of prompt photon processes is that the observed final-state photon emerges from the QCD process directly, without the intermediate hadronisation that accompanies the observation of a quark or gluon through the means of a final state jet. On the other hand, the cross sections are substantially lower than those of dijet processes. Both BH photons interact in a direct way in producing the lepton pair.
Broad arrows represent photon or photon remnants.
The study of lepton pair production through DY processes is of interest because it can provide a further alternative way to measure the parton densities of the photon and the proton, testing perturbative QCD and the determination of the running coupling constant. Much attention was paid to find proper kinematic variables and to determine a kinematic region where the major contribution from the DY leptons would not be dominated by the background processes. All of them, however, reached the not very comforting conclusion that, because of the overwhelming Bethe-Heitler background, photoproduction of lepton pairs from a proton target is not an adequate way of measuring the photon structure function through the DY effect. The planned HERA upgrade, however, will increase the integrated luminosity by two orders of magnitude and allow stricter selection criteria to separate the processes of interest. One should therefore make an effort to analyse the Drell-Yan and background processes using a more precise approach to the experimental conditions.