Calibrating the CERN ATLAS Experiment with $E/p$

Inside the ATLAS experiment two proton beams will collide with a center of mass energy of 14 TeV. These proton beams will be delivered with unprecedented high collision rates by the Large Hadron Collider (LHC) at the European Center of Particle Physics, CERN. For important parts of the physics program of ATLAS, e.g. the search for the Higgs boson, the performance of the electromagnetic calorimeter, whose primary task is to measure the energy of electrons and photons, is crucial. The main topic of this thesis is the intercalibration of the energy scale of the electromagnetic calorimeter and the momentum scale of the inner detector. This is an important consistency test for these two detectors. The intercalibration is performed by investigating the ratio E/p for electrons, i.e. the ratio of the energy E measured by the electromagnetic calorimeter and the momentum p measured by the inner detector. The starting point is the Combined Test Beam (CTB) 2004, where a segment of the ATLAS detector was exposed to different particle beams with different energies, ranging from 1 GeV to 350 GeV. First, I have investigated a calibration procedure using Monte Carlo simulation for the energy measured by the electromagnetic calorimeter for electrons. The performance of this procedure is presented for data taken in the CTB 2004. Second, I have developed a model for E/p which allows the disentanglement of the ratio of the two scales from tail ef fects from the different detector response functions of the inner detector and the electromagnetic calorimeter. The performance of this model for intercalibration is shown for the Monte Carlo simulation for the CTB 2004 and compared to data taken in the CTB 2004. Finally I have evaluated the performance of this method for the full ATLAS detector using Monte Carlo simulation. Although the energy scale of the electromagnetic calorimeter will ultimately be determined with electron/positron pairs from Z boson decays, the potential of the intercalibration method with initial data, and therefore limited statistics, is presented. With the presented intercalibration method the energy scale can also be determined for various electron energies, thereby measuring the linearity of the electromagnetic calorimeter in situ. This will only be limited by statistics, i.e. the number of electrons produced at high energies, and ultimately the capability of the inner detector to measure the momentum of charged particles at very high energies.