Jet Cross Sections and Jet Shapes in Photoproduction

The main source of jets at HERA a comes from collisions between protons and quasi-real photons (Q ≈ 0, where Q is the virtuality of the photon) emitted by the electron beam (photoproduction). At lowest order QCD, two hard scattering processes contribute to jet production : the resolved process, in which a parton from the photon interacts with a parton from the proton, producing two jets in the final state; and the direct process, in which the photon interacts pointlike with a parton from the proton, also producing two jets in the final state. The study of high-pT jet photoproduction provides tests of QCD and allows to probe the structure of the photon. In perturbative QCD the cross section for jet production is given by