Kalibration des ALICE Übergangsstrahlungsdetektors und ein Studium der Z-Boson und schwere Quarks Produktion in pp Kollisionen an der LHC

The ALICE Experiment is one of the four experiments installed at the Large Hadron Collider (LHC). One of its detector-systems, the Transition Radiation Detector (TRD), is a gas detector designed for electron identification and charged particle tracking. The charged particle ionizes the gas along its path and electrons drift in an uniform field of 700 V/cm over 3 cm before being amplified. We implemented procedures to calibrate the drift velocity of the electrons, the time-offset of the signal, the amplification factor and the width of the Pad Response Function (PDF) characterizing the sharing of the deposited charge over adjacent pads. Physics events (pp and PbPb collisions) will be used. The performances of the algorithms were tested on simulated pp collisions at √s=14 TeV and on first real data taken with cosmic-rays in the ALICE setup. The calibration software was installed on the Data Acquisition System at CERN and executed continuously during the cosmic-ray data taking in 2008, providing a first determination of the calibration constants. This thesis presents also a study on the capability of the ALICE central barrel to detect the Z-boson through the decay Z→e+e- in pp collisions at 14 TeV. The production cross-section has small theoretical errors in QCD and a comparison between the experimental results and the theoretical calculations allows to check the understanding of the detector response at high transverse moment um. We demonstrated that the Z→e+e- is characterized by a very clean signal in the dielectron reconstructed invariant mass spectrum. At such high transverse momentum (about 45 GeV/c=mZ/2), the electrons from Z are identified with the Transition Radiation Detector. The remaining background from misidentified pions and electrons from heavy-flavored decays are rejected by the requirement of two isolated reconstructed tracks. The main challenge comes from the very small production rate. Therefore we estimated the efficiency of a trigger based on a low pT cut and electron identification with the TRD and showed that about 100 Z→e+e- can be reconstructed per year employing such a trigger. Another physics topics investigated in this thesis is the measurement of the charm and bottom production via their semileptonic decays. These measurements allow at high pT to test pQCD calculations in pp collisions and are essential prerequisites for the understanding of the Quark Gluon Plasma expected to be produced in PbPb collisions. In this work, we studied the electrons from c and b decays reconstructed in the central barrel. For 108 minimum-bias events at √s=10 TeV (as was initially expected for the year 2008), a pT of about 6 GeV/c can be reached. The electrons are identified with the Time Projection Chamber, TRD and Time-Of-Flight. First estimations of the contamination and Particle Identification efficiency based on a Bayesian approach were performed. At high pT, electrons from heavy-flavored hadrons become dominant but at low pT the main source of electrons is gamma conversion in the detector material and the π0 Dalitz decay. We showed that the expected signal-to-background ratio, which was found to be similar as in the PHENIX experiment for pp collisions at √s=200 GeV, will allow a measurement of the charm and bottom cross-sections.