Single-electron analysis and open charm cross section in proton-proton collisions at $\sqrt{s}$ = 7 TeV

The Large Hadron Collider (LHC) at CERN is the world’s highest energy hadron collider, providing protonproton collisions currently at a centre-of-mass energy $\sqrt{s}$ = 8 TeV and Pb-Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV. This opens a new energy regime, which allows the study of QCD in elementary pp-collisions and in the extreme environment of Pb-Pb collisions, as well as providing a discovery potential for rare and exotic particles. ALICE is the dedicated heavy-ion experiment at the LHC. The experiment is optimised to provide excellent tracking and particle identification capabilities, in particular at low-$p_{t}$, where the bulk of the particles is produced in heavy-ion collisions as well as in proton-proton collisions. The production of heavy quarks is described in proton-proton collisions by next-to-leading order perturbative QCD (pQCD) calculations. Thus, the measurement of heavy-quark production in proton-proton collisions serves as a test of pQCD. Measurements performed at SPS, RHIC, and Tevatron experiments showed a good agreement with pQCD, where the data were usually at the upper limit of the prediction. In addition, measurements in proton-proton collisions serve as reference for heavy-ion collisions, in which heavy quarks are essential probes for parton energy loss in a deconfined medium. Heavy-quark production can be studied either with hadronic or in semi-leptonic decay channels. The analysis presented in this thesis is performed in the semi-electronic decay channel with the ALICE apparatus. A crucial device for the electron selection is the Transition Radiation Detector (TRD), which provides an important contribution to the electron-pion separation for momenta larger than 1 GeV/c. In November 2010, the first data were recorded with the experiment. The electron selection performance was studied for the first time on real data using data-driven methods. A pion-rejection factor of 23 at a momentum of 2 GeV/c was obtained using a likelihood method on the total charge deposit in the detector for tracks with the maximum amount of charge deposit measurements. The inclusive electron $p_t$-spectrum, which contains contributions from heavy-flavour hadrons as well as from various background sources, was measured for 0.5 GeV/c < $p_t$ < 8 GeV/c in proton-proton collisions at $\sqrt{s}$ = 7 TeV at midrapidity (|y| < 0.5). The contribution of background electrons was quantified using a cocktail method, and it was subtracted from the inclusive spectrum. For the resulting spectrum of electrons from heavy-flavour hadron decays a signal-to-background ratio of 1 was observed at $p_t$ = 2 GeV/c. This ratio grows with increasing electron $p_t$ up to ~5 at $p_t$ = 8 GeV/c. The $p_t$ differential cross section of electrons from heavy-flavour hadron decays obtained by this method is in good agreement with fixed-order plus next-to-leading logarithm pQCD (FONLL) predictions. The total charm cross section in proton-proton collisions obtained from this analysis is $\sigma_c$ = 7.6 $\pm$ 0.3(stat) $\pm$ 2.9(sys)$^{+3.2}_{-2.5}$ (extr) $\pm$ 0.3(norm)$\pm$ 0.3(br) mb. Results from this analysis are published in [1].