Threshold Resummation Studies for Polarized and Unpolarized High-pT Hadron Production at COMPASS

This thesis is focused on threshold resummation studies for high-pT inclusive-hadron production in longitudinally polarized lepton-nucleon scattering at COMPASS. The goal is to address threshold resummation effects to double-longitudinal spin asymmetries at COMPASS kinematics. At these kinematics, nearly all available energy is used for the production of the high-pT parton and its recoiling counterpart. In that case, the phase space for additional radiation of partons becomes small, allowing only soft and/or collinear gluon radiation which results in large logarithmic QCD corrections at every order in perturbation theory. These logarithmic contributions spoil the perturbative expansion, and thus have to be resummed. Using threshold resummation techniques in Mellin-N momentum space, we are able to deal with those logarithmic corrections up to all perturbative orders of QCD at a certain logarithmic level. In our calculations we choose next-to-leading logarithmic accuracy. Further, we develop a framework to include subleading 1/N-suppressed logarithms into future calculations. In our phenomenological results for COMPASS we present a detailed study of the impact of next-to-leading logarithmic threshold resummation on the spin-dependent cross section and on the corresponding double-longitudinal spin asymmetry for the high-pT photoproduction process. We include resummation for the direct, as well as for the resolved-photon contribution. In a comparison of the spin-averaged with the spin-dependent cross sections we find out that the latter receives smaller corrections from resummation than the spin-averaged one, indicating that threshold corrections do not cancel in the double-spin asymmetry and rather tend to decrease it, yielding an overall better agreement between experiment and theory. Finally, we reveal that the parton-to-hadron fragmentation functions have a strong impact on the size and shape of the predicted spin asymmetries