In QPM there is a 1+1 parton configuration, fig. 2a, which consists of a single struck quark and the proton remnant, denoted by ``+1''. At HERA energies there are significant higher-order Quantum Chromodynamic (QCD) corrections: to leading order in the strong coupling constant, these are QCD-Compton scattering (QCDC), where a gluon is radiated by the scattered quark and Boson-Gluon-Fusion (BGF), where the virtual boson and a gluon fuse to form a quark-antiquark pair. Both processes have 2+1 partons in the final state, as shown in fig. 2. There also exists calculations for the higher, next-to-leading (NLO) processes.
Perturbative QCD does not predict the absolute value of the parton densities within the proton but determines how they vary from a given input. For a given initial distribution at a particular scale Altarelli-Parisi (DGLAP) evolution [9] enables the distributions at higher Q2 to be determined. DGLAP evolution resums the leading contributions associated with a chain of gluon emissions. At large enough electron-proton centre-of-mass energies there is a second large variable 1/x and, therefore, it is also necessary to resum the contributions. This is acheived by using the BFKL equation [10].
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