Studies at the CMS experiment on silicon microstrip module defects and on photon identification in semileptonic ttbar-events

The present thesis has been accomplished in the framework of the Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC). It is divided into two parts. The first one describes the studies that have been performed by a centralized Module Debug Centre to investigate the silicon microstrip detector modules for the CMS inner tracker endcaps that have not passed the qualification procedure during the mass production. A scheme of seven global defect classes has been introduced. The signatures of each class as well as the methods to sort a module into a certain class are illustrated. In total, 225 modules have been examined and 126 of them could be repaired. No systematic source of defects has been observed for the entire production phase. The second part deals with the identification of semileptonic t¯t-events (electron and muon channel) containing photons that are radiated off top-quarks. A signature based Monte Carlo analysis reveals that the separation of the t¯t-events from the non-t¯t background (W + Jets, Z + Jets, di-boson, + Jets, and QCD-multijet events) is possible. The resulting photon energy spectra are dominated by photons radiated off top-quarks. The situation changes considerably if furthermore an inclusive t¯t + Jets data-set is included. The semileptonic fraction has the same event signature as the signal events. Due to the irreducible background from the decay 0 ! possibly being part of the hadronic structure of these events the energy spectra are now mainly formed by the latter photons. A signal-to-background ratio of 1/46 (electron channel) and 1/31 (muon channel) can be achieved. If the t¯t + Jets events are not considered as background but replace the signal data-set because of a 10% to 15% admixture of photons that are radiated off top-quarks in these events, a first study on the distinction between a top-quark with an electromagnetic charge of 2/3e and −4/3e has been implemented. It is shown that the potential exists to directly measure the value of this electromagnetic coupling parameter of the top-quark with a few 10 fb-1 of LHC collision data at the CMS experiment.