Measurement of the High Mass Drell-Yan Differential Cross Section in the electron-positron channel with the ATLAS Experiment at $\sqrt{s}$ = 7 TeV

This thesis reports the study of the stability of the beam pipe within the ATLAS experiment and of the high mass Drell-Yan $Z/gamma^{ast} o e^+e^-$ differential cross section measurement using 4.92 fb$^{-1}$ of ATLAS data recorded in 2011 at a centre-of-mass energy of 7 TeV. The beam pipe studies used reconstructed secondary hadronic interaction vertex distributions in the 2010-2012 data. The results indicate a very good stability and will used for the insertion of the new beam pipe. A simulation study for the design of the new beam pipe has been conducted in addition to estimate the influence of a change of material and the difference between three options for the position of a new vacuum flange in the beam pipe inside the electromagnetic calorimeter end-caps. A gain of 27 % in radiation length was found when replacing the stainless steel beam pipe with aluminium but no significant difference between the three flange positions could be observed. These results have been used by the ATLAS engineering for the design of the new beam pipe that will be installed during the 2013/14 LHC shutdown. In order to perform the Drell-Yan cross section measurement the identification efficiency of electrons has been evaluated up to a transverse energy of 500 GeV. The results are presented separately in bins of pseudorapidity $eta$ and transverse energy E$_{ m T}$ and compared to previous measurements for E$_{ m T} <$ 50 GeV. Good agreement is found. At high E$_{ m T}$ the data agrees well with the Monte Carlo simulations. The Drell-Yan differential cross-section is reported as a function of the electron-positron invariant mass, mee, for events with $116 < m_{ m ee} < 1500$ GeV in a fiducial region with $e^{pm}$ pseudorapidity of $| eta | <$ 2.5, and transverse energy E$_{ m T} > 25$ GeV. The precision of the measurement at high m$_{ m ee}$ is dominated by the statistical uncertainty on the data. The results of the differential cross section measurement are compared to the predictions of perturbative next-to-next-to leading order QCD calculations and various event generators. The data are largely consistent with the theoretical predictions.