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Dedek, Nicolas (2005): Photon-Gluon-Fusion zu D + antiD -Mesonen am COMPASS-Experiment. Dissertation, LMU München: Faculty of Physics
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Abstract

One of the main goals of the COMPASS experiment at CERN is the measurement of the gluon contribution DeltaG to the nucleon spin. A longitudinal polarised muon beam with 160 GeV energy is used. A 6LiD target can be polarised parallel and antiparallel with respect to the muon spin. In the photon gluon fusion (pgf) a virtual photon emitted from the muon interacts with a gluon from a target nucleon via the creation of a quark-antiquark pair. A pgf event can be identified through the detection of the creation of heavy quarks, e.g. through the reconstruction of charmed D mesons. The cross section of the pgf with the creation of D mesons depends on the polarisation of the photon and the gluon. DeltaG/G can be extracted from the counting rate asymmetry of the D mesons created with the different polarisations of the target. In this work D mesons were reconstructed in different decay channels. The number of reconstructed D mesons and the signal to background ratio in the signal region were used to estimate the statistical error of DeltaG/G. Different kinematic cuts which suppress the background were optimized with respect to the error of DeltaG/G. The main contribution to the measurement of DeltaG/G comes from the golden D* channel (D*+ -> D0 pi+ -> (K- pi+)pi+ and respectively D*- ->D0bar pi- -> (K+ pi-)pi-). The golden D0 channel (D0 -> K- pi+ and respectively D0bar -> K+ pi-) has a high background with contributions of some very broad K* resonances. It is difficult to determine the strengths of these resonances in the Kpi mass spectrum. The resulting systematic error still has to be investigated. In total the statistical error of DeltaG/G amounts to 1.45 (using 2002 and 2003 data). The reconstruction probability of charged tracks as a function of momentum and transverse momentum as well as the reconstruction efficiency of K0s mesons and the identification efficiency of charged kaons was determined with Monte-Carlo simulations. These efficiencies were checked in the data comparing 2 decay channels of the charged D* resonance and reconstructing pairs of D mesons. Using these efficiencies the contribution of other decay channels to the DeltaG/G measurement could be estimated. Three channels are proposed. Their contribution corresponds each to 7 - 28 % more data in the golden D* channel.