D-meson production by muons in the COMPASS experiment at CERN

One of the physics goals of the COMPASS experiment at CERN was to measure the contribution of gluons to the nucleon spin. To achieve this, it was proposed to scatter polarized 160 GeV=c muons on a polarized deuteron target and to detect D mesons in the nal state. The underlying process in this D meson production is supposed to be the Photon-Gluon Fusion (PGF), where a virtual photon emitted by the muon interacts with a gluon from the target nucleon, producing a charm-anticharm quark pair. Fragmentation of a charm (anticharm) quark leads with high probability to the creation of a D0 or D meson, which COMPASS detects via the D0 ! K and D ! D0 ! K decay modes. From the longitudinal cross section spin asymmetries of the D meson production and theoretical predictions for the PGF cross section, the gluon contribution to the nucleon spin has been measured by the COMPASS experiment. The results presented in the thesis are the following. Based on data from the year 2004 a total visible cross section of 1:8 0:4 nb, for the D meson production, has been measured, with the error being dominated by systematic eects. It is validated that the D mesons are indeed produced through the PGF process, by comparison of measured D meson kinematic distributions to the ones predicted by a theory (AROMA generator). A good agreement was found for the distribution shapes, which conrms that PGF plays a major role. However, a 20% dierence was found in the number of produced D0 and D0 mesons (and for the D+ and D􀀀 mesons as well) which is signicantly larger than predicted by AROMA. Kinematic distributions of D0 and D mesons were compared with the background and also with the nearby K 2 (1430)0 resonance, using all longitudinal data taken in 2002-2006. The particle-antiparticle asymmetry has been studied as a function of several kinematic variables. The 20% excess of mesons decaying into K+􀀀 over mesons decaying into K􀀀+ was observed for all three mesons. The behavior of the D0=D0 (and D+=D􀀀) asymmetries as a function of virtual photon energy suggests that associated production of D􀀀+c or D0+c may be responsible for the observed eect. The longitudinal double spin asymmetries have been studied for the D0, D and K 2 (1430)0 mesons separately for particle, antiparticle and for the sum of particle and antiparticle. It was found that the asymmetries extracted for D0 and D mesons are compatible with zero. A 3-sigma deviation from zero asymmetry was observed for the K 2 (1430)0 meson. An investigation of the K 2 (1430)0 double spin asymmetry reveals a dependence as function of the Bjorken xBj variable.