2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/45803Quantum field theory near thresholds of ultra-heavy particles is considered. We focus especially on the top-antitop and the stop-antistop systems. The question of the possible large corrections due to a running width of the top quark could be solved in this work. It was shown that a previously not considered contribution gives rise to large cancellations and leads especially to a gauge independent result. With the help of a new zero order equation and a Ward identity it was further possible to derive a general therorem for bound state corrections to the width. Furthermore a systematic derivation of the top-antitop and the stop-antistop potential to numerical order O(α_s^4) was given. In the second part of this work we consider within the numerical Green function approach the cross sections for the production of ultraheavy particles near threshold. The forward-backward asymmetry and the pure axial contribution to the top-antitop total cross section as well as the stop-antistop total cross section are calculated. A systematic method is given to avoid unnatural cut-offs in the nonrelativistic treatment.thesis, 84 pages, uuencoded latex files and figuresHigh Energy Physics - Phenomenologytext