Aspects of QCD and the Photon Structure

Abstract: In a single theoretical framework, real and virtual photon interactions are modeled for gamma-p and gamma-gamma collisions. The description covers aspects from the production of jets to total cross sections, where special emphasis is put on the range of uncertainty in the modeling of a resolved photon component. Our approach extends on a model for photoproduction, where the total cross section is subdivided into three distinct event classes: direct, VMD and anomalous. It provides a smooth transition between the region of real and highly virtual photons. In jet production, photon flux factors are convoluted with matrix elements involving either direct or resolved photons and, for the latter, with parton distributions of the photon. Effects of resolved longitudinal photons are estimated by simple multiplicative factors on the resolved transverse-photon contributions and their impact on some representative distributions are investigated. Complete events are generated with Monte Carlo methods and results are confronted with data from specifically HERA and LEP2. By the breakdown into different event classes, we aim for a complete picture of all event properties in gamma-p and gamma-gamma collisions. When there are identical colour charges in a parton cascade, there are several ways to stretch a string, or a cluster chain, in the subsequent hadronization phase. In particular, the possibility that a colour singlet gluon system hadronizes isolated from the remainder of the system in hadronic Z decays is studied. QCD aspects of leptoquark production at HERA are discussed with focus on initial- and final-state QCD radiation. By taking into account corrections to this, in mass reconstructions of a leptoquark, a narrower mass peak and a better average mass value is obtained.