The Energy Response of the ATLAS Calorimeter System

The Large Hadron Collider (LHC) currently under construction at the European Organization for Nuclear Research (CERN) in Geneva will collide two proton beams with a center-of-mass energy of 14 TeV. At this high energy frontier a new chapter of particle physics will be opened. The ATLAS experiment is a general-purpose LHC detector for proton-proton collisions. The electromagnetic liquid argon-lead sampling calorimeter (LAr Calorimeter) is designed to measure the energy and position of electrons and photons with high precision and the hadronic scintillator-iron sampling calorimeter (TileCal) complements the measurement of the energy and direction of jets. Both calorimeters are installed in the ATLAS experimental cavern and are presently being commissioned. To be able to start the commissioning of the TileCal in an early phase, even before the final electronic readout system was available, a mobile data acquisition system (MobiDAQ) was developed in the context of this PhD-thesis. It is capable of reading up to eight TileCal modules and performs systematic tests to verify the electronics. In addition it can record data from cosmic ray muons in the ATLAS experimental cavern. The muon data from the cosmic ray measurements taken with the MobiDAQ system are analyzed. The typical energy distribution of the muons is reconstructed. The time resolution in TileCal is determined to be about 2 ns and it is shown that it is possible to synchronize the cells of the LAr Calorimeter with cosmic muons using TileCal as reference. The main part of the thesis investigates to what extent muons can be used to calibrate the calorimeter cells and to establish an absolute energy scale. Muons traversing the calorimeters leave a well defined energy deposit that can serve as a reference signal. It is shown that the mean measured energy of muons can be understood at the percent level. Muon data in the energy range of 20 to 350 GeV were obtained in a testbeam where a slice of the ATLAS detector was installed. A Monte Carlo simulation was used to study in detail the dependence of the muon energy deposition on the muon energy, the cell geometry and the particle impact point. The mean measured data are about 2% lower than the simulation for all calorimeter layers and for muon energies between 20 and 150 GeV. Good agreement between the LAr calorimeter and the TileCal is found. This is a remarkable accuracy on the absolute energy scale. The systematic uncertainty, as estimated from the spread of the measurement in different layers and for different energies, is less than 1.5% The mean measured energies are corrected for several detector and reconstruction effects. The resulting true deposited energy is compared to a calculation using first principles of energy lost by muons. An agreement of 4% is observed.