Higher order QCD studies of semi-inclusive deep-inelastic scattering

In this dissertation, I worked intensively on the calculation of contributions to the cross sections of semi-inclusive deep inelastic scattering (SIDIS). The obtained results can be used to obtain information about the parton distribution functions and fragmentation functions from SIDIS data. They thus contribute to a more precise determination of the nucleon structure. In particular, these findings also lead to information about the spin structure of the proton. In the context of perturbative quantum chromodynamics, contributions to the perturbation series of the transversal structure function in SIDIS, which occur in all orders of the ongoing strong coupling, were investigated. This phenomenon, known as threshold resummation, occurs in the region of phase space where logarithmic contributions diverge to a fixed order. Summing those logarithmic contributions in all orders ultimately yields finite contributions to the structure function. Knowledge about the structure of such contributions makes it possible to predict them to an arbitrary fixed order. This makes it possible to approximate the structure function in a fixed order using so-called threshold logarithms. The aim of the present dissertation is to obtain an approximation of the transversal structure function in the second-leading order using threshold logarithms. (Leading) contributions that are suppressed in the threshold region of the phase space were also taken into account. In the last part of the dissertation, the behavior of the SIDIS structure functions for the scattering of longitudinally polarized protons with unpolarized electrons at small transverse momenta is investigated. In this area, SIDIS data can be described particularly well by transverse momentum-dependent parton distribution functions. Here, contact with those distribution functions from the side of collinear factorization is established and the so-called T-odd asymmetry in the limit of small transversal momenta is considered.