Messung der Charmonium-Produktion und Energiekalibration für Elektronen mit dem Atlas-Experiment

The cross section of Charmonium production was measured using data from pp collisions at s^{1/2}=7TeV taken by the Atlas experiment at the LHC in 2010. To improve the necessary knowledge of the detector performance, a calibration of the energy was performed. Using electrons from decays of the Charmonium, the energy scale of the electromagnetic calorimeters was studied at low energies. After applying the calibration, deviations in the energy measurement were found to be lower than 0.5% by comparing with energies determined in Monte Carlo simulations. With an integrated luminosity of 2.2 pb^{−1}, a first measurement of the inclusive cross section of the process pp->J/psi(e^+e^−)+X at s^{1/2}=7TeV was done. For this, the accessible region of transverse momenta p_{T,ee}>7GeV and of rapidities |y_{ee}|<2.4 was used. Differential cross sections for the transverse momentum p_{T,ee}, and for the rapidity |y_{ee}| were determined. Integration of the differential cross sections yields the values (85.1±1.9_{stat}±11.2_{sys}±2.9_{Lum})nb, and (75.4±1.6_{stat}±11.9_{sys}±2.6_{Lum})nb for sigma(pp->J/psiX)BR(J/psi->e^+e^−), being compatible within systematics. Comparisons with measurements of the process pp->J/psi(μ^+μ^−)+X done by Atlas and CMS have shown good agreement. To compare with theory, predictions from different models in next-to-leading order, and partially considering contributions in next-to-next-to-leading order were combined. Comparisons show a good agreement when taking into account contributions in next-to-next-to-leading order.