Jets and QCD description of jet events
At HERA photoproduction can be studied in inelastic electron proton reactions at very small momentum transfers from the electron to the proton. In photoproduction the photon (radiated from the electron) may fluctuate into quark pairs (and these can radiate gluons). This can be calculated by ascribing parton densities to the photon (The parton densities describe here the probability to find a parton in the photon). The paper describes a very precise measurement of 2-jet cross sections in photoproduction. This data are the first jet measurements published by the H1 collaboration with a significant reduced jet energy scale uncertainty. The data were compared to leading and next-to-leading order QCD calculations. It was found that the data are describable by next-to-leading order QCD calculations without significant changes in the parameterisations of parton densities and that the data are useful to further constrain the existing ones. The work increased the understanding of jets with high transverse momentum and made further searches for deviations from the SM in multijet final states possible.