Alignment of the ATLAS Inner Detector tracking system

The Large Hadron Collider (LHC) at CERN is the world's largest particle accelerator. After a successful start run at 900 GeV in 2009, during 2010, LHC will collide two proton beams at an unprecedented centre of mass energy of 7 TeV. ATLAS is one of the four multipurpose experiments that will record the products of the LHC proton‐proton collisions. ATLAS is equipped, among others, with a charged particle tracking system built on two different technologies: silicon planar sensors and drift‐tube based detectors constituting the ATLAS Inner Detector (ID). In order to achieve its scientific goals, ATLAS has quite exigent tracking performance requirements. Thus, the goal of the alignment is set such that the limited knowledge of the sensors location should not deteriorate the resolution of the track parameters by more than 20% with respect to the intrinsic tracker resolution. In this manner the required precision for the alignment of the silicon sensors in its most sensitive direction is below 10 micrometers. The alignment of the ATLAS tracking system requires the determination of almost its 36000 degrees of freedom with high accuracy. This demands to use a large sample of high momentum and isolated charge particle tracks. A track based alignment approach has been implemented in the ATLAS software framework as well as a description of the detector geometry that accounts for assembly structures and individual modules. A frequency scanning interferometer (FSI) system is also run to monitor the stability of the silicon strip detector structures. Data with FSI system has been recorded and analyzed and show how stable the detector is and its response to events that affect the detector operation. We will present the outline of the track based alignment approaches, their code implementation within the ATLAS computing framework and their results when aligning the real detector. So far the proposed alignment algorithms have been applied on the real data recorded from both: the LHC collisions and cosmic ray data as well. Results will be shown using 2009 and 2010 LHC data. Finally the impact of the alignment on physics measurements will be discussed.