Hadron response and shower development in the ATLAS calorimeters

Abstract 1 In the year 2004 a full slice of the Atlas barrel detector has been exposed to a pion test-beam covering the momentum range between 2 and 350 GeV. We report on results from a scan of beam impact points from eta=0.2 to eta=0.65 for beam momenta from 20 to 350 GeV in a set-up with the electro-magnetic (Lar/Pb and the hadronic calorimeter (Tile/scintillator) operational, and, on the results of a data taking period were, in addition, the inner detector system was operational and pion momenta from 2 to 180 GeV. The mean energy, the energy resolution and the longitudinal and radial shower profiles, and, various observables characterizing the shower topology in the calorimeter are measured using the same reconstruction method as used for the analysis of proton-proton collisions. The Monte Carlo simulation based on the Geant4 program is able to reproduce the pion response within a few percent. The largest deviations (up to 5%) are observed in the low pion momentum range. The simulation predicts a resolution that is narrower than found in the data. The hadronic showers are narrower and shorter than the ones measured in the data. Abstract 2 The response of pions and protons in the energy range of 20 to 180 GeV, produced at CERN's SPS H8 test beam line in the ATLAS iron-scintillator Tile hadron calorimeter, has been measured. The test beam configuration allowed the measurement of the longitudinal shower development for pions an d protons up to 20 nuclear interaction lengths. It was found that pions penetrate deeper in the calorimeter than protons. However, protons induce showers that are wider laterally to the direction of the impinging particle. Including the measured total energy response, the pion-to-proton energy ratio and the resolution, all observations are consistent with a higher electromagnetic energy fraction in pion-induced showers. The data are compared with GEANT4 simulations (G493) using several hadronic physics lists. The Monte Carlo simulation is able to reproduce the pion and the protons response within a few percent. The resolution is found to be wider in the data. In general, pions are better described by the simulation than protons.