Measurement of open charm in 158 AGeV/c Pb - Au collisions

This thesis presents a measurement of an upper limit for the open charm yield in 158 AGeV/c Pb-Au collisions with the CERES spectrometer at 7% centrality. A secondary particle reconstruction scheme, based on the reconstruction of the decay vertex, is developed and tested using the decay K0short -> Pi+ Pi- as a reference measurement. An integrated K0short rapidity density of dN/dy = 19.75 +- 0.23(stat) +- 1.70(syst) is measured in the rapidity region 2.0 < y < 2.6 and compared to results from an alternative analysis of the CERES data and results from the NA49 and NA57 collaborations. The analysis of the decay D0bar -> K+ Pi- requires careful study of the combinatorial background and resonances contributing to the invariant mass spectrum. An open charm enhancement of more than a factor 22 can be excluded at 98% confidence level. The enhancement is calculated with respect to the expected open charm yield in nucleus-nucleus collisions of <D0bar> = 0.21 per event, obtained by scaling the charm cross-section in proton-proton collisions with the number of binary collisions. The first part of this thesis is devoted to the development of a hit finding algorithm for the CERES TPC. Furthermore, a detailed description of the calibration of the TPC is presented. A position resolution of sigma_(r Dphi) = 340 um and sigma_(Dr) = 640 um is achieved with the new calibration, with a momentum resolution of Dp/p = 5.4% at p = 5 GeV/c, the momentum region relevant for the D0bar meson.