Energy Measurement of Hadrons with the CERN ATLAS Calorimeter

The ATLAS detector is a multi-purpose detector measuring the energy and direction of particles produced in proton-proton collisions at a center of mass energy of 14 TeV provided by the Large Hadron Collider at the European center of particle physics, CERN. The main aim of this thesis is to assess the precision of the present understanding of the interactions of hadrons with matter (as implemented in Monte Carlo (MC) simulations) to describe the response of the ATLAS calorimeter and to predict the correction necessary to measure the full energy of pions. The simulations are compared to testbeam data. The present description of the response of the ATLAS central calorimeter is able to predict the energy corrections, as verified by using testbeam data. For the Combined Testbeam 2004 (CTB) a full slice of the central region of the ATLAS detector including all sub-detectors has been installed in the H8 beam line of the CERN SPS accelerator. Pions and electrons with the energies ranging from 1 to 350 GeV have been measured. The ability of the various MC simulations to describe the basic observables has been tested in the CTB. It has been found that the longitudinal shower development is described by the fragmentation model, while the response and the radial shower extension is well represented by using intra-nuclear cascade models. Using the best model, the description of the response is within 1% for a beam energy of 20 GeV and abov e. The mean energies in the individual calorimeter layers are described within 10 to 20%. The hadronic calibration scheme has been applied on MC and data. It has been shown that the fully corrected pion signal is within 2% (5%) of the initial pion energy for E>20 GeV in MC (data). The resolution is improved by about 10 (10) to 15% (40%) compared to the non-corrected signal in data (MC). The MC simulation has problems to describe the measured energy resolution. The deviation of MC and data is expected to become smaller when the corrections for the hadronic calibration are derived with the new version of GEANT4 (the MC simulation framework) . Beyond the ATLAS standard pion calibration scheme a novel technique for the correction for the energy loss due to dead material has been developed which has shown to improve the linearity and the resolution.