Comparison of data with Monte Carlo simulations at the ATLAS barrel Combined Testbeam 2004

The scheme adopted as baseline by ATLAS for the calibration of hadrons depends strongly on the quality of the description of the data by simulations. In 2004, the calorimeters of the ATLAS barrel region have been exposed to a testbeam in order to evaluate the energy response of pions for the energies ranging from 1 to 350 GeV. For the energy region from 3 to 9 GeV a data analysis with the full systematic uncertainty is available. The data has been compared extensively to GEANT4 simulations. Several combinations of physical models-the so called "physics lists"-are provided by the GEANT4 collaboration and have been evaluated. The best overall description of data is achieved with the physics list QGSP_BERT which describes the energy response of pions within a few percent. QGSP_BERT has been adopted by ATLAS for the simulation of the first data.