Muon Detector-Description as-built and its Simulation for the ATLAS Experiment

The ATLAS detector at CERN is designed to fully exploit the physics potential of the LHC by providing precision measurements of the mass and identification of particles emerging from high-energy proton-proton collisions. The ATLAS Muon Spectrometer is a large and complex system, comprising Monitored Drift Tubes (MDT), Cathode Strip Chambers (CSC), Resistive Plate Chambers (RPC), and Thin Gap Chambers (TGC). These systems are described in the software using a set of geometrical primitives, called GeoModel, which constructs a transient model from primary parameters, called AMDB, stored in the detector description database. The model provides a single interface to the GEANT4 simulation, event reconstruction, and trigger software. In the present note we describe the current status of the ATLAS Muon Spectrometer simulation, the geometry software, placing an emphasis on the achievement of realistic effects recently implemented to take into account the detector as-built. A first evaluation of as-built geometry in the simulation on the Spectrometer performance and on the physics observable is also discussed.