Commissioning of the ATLAS Reconstruction Software with First Data

Looking towards first LHC collisions, the ATLAS detector is being commissioned using all types of physics data available: cosmic rays, beam-halo and beam-gas events produced during the expected LHC single beam operation period prior to proton collisions. In addition to putting in place the trigger and data acquisition chains, commissioning of the full software chain is a main goal. This is interesting not only to ensure that the reconstruction, monitoring and simulation chains are ready to deal with LHC physics data, but also to understand the detector performance in view of achieving the physics requirements. Cosmic rays have allowed us to study the ATLAS detector in terms of efficiencies, resolutions, channel integrity and alignment and calibrations. They have also allowed us to test and optimize the muon combined performance algorithms. Single beam events will allow increasing the statistics in the endcap region and to study the detector performance at the expected LHC beam crossing rate.