Alignment of the ATLAS Inner Detector Tracking System

ATLAS is a multipurpose experiment that records the products of the LHC collisions. In order to reconstruct trajectories of charged particles produced in these collisions, ATLAS is equipped with a tracking system built on silicon planar sensors (Pixels and micro-trips) and drift-tube based detectors. They constitute the ATLAS Inner Detector. It contains 1744 pixel modules (1456 in 3 barrel layers and 288 in 6 end cap discs). The pixel size is 50x400 µm2 . In order to achieve its scientific goals, the alignment of the ATLAS tracking system requires the determination of its almost 36000 degrees of freedom (DoF) with high accuracy. Thus the demanded precision for the alignment of the pixel and micro-trip sensors is below 10 µm. This implies to use a large sample of high momentum and isolated charge particle tracks. The high level trigger selects those tracks online. Tracks from cosmic trigger during empty LHC bunches are also used as input for the alignment if they cross the pixel detector volume. The implementation of the track based alignment within the ATLAS software framework unifies different alignment approaches and allows the alignment of all tracking subsystems together. The pixel measurements have a major role in the primary vertex and beam spot constraints. The pixel module assembly survey is also used as starting geometry. As alignment algorithms are based on minimization of the track-hit residuals, one needs to so lve a linear system with large number of DoF. The alignment jobs are executed at the CERN Analysis Facility. The event processing is run in parallel in many jobs. The output matrices from all jobs are added before solving. We will present the results of the alignment of the ATLAS tracker using real data recorded during cosmic commissioning phases in 2008 and 2009, the LHC start up run in 2009 and the recent 7 TeV data collected during 2010 run. The results allow a detailed study of the pixel modules geometry. The results of the alignment with real data reveal that the precision of the alignment constants is just below 10 µm.