Alignment of the ATLAS Inner Detector

In order to reach the track parameter accuracy requested by the physics goals of the experiment, the ATLAS tracking system requires to determine accurately its almost 700,000 degrees of freedom. The demanded precision for the alignment of the silicon sensors is below 10μm. 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 alignment software counts of course on the tracking information (track-hit residuals) but also includes the capability to set constraints on the beam spot and primary vertex for the global positioning, plus constraints on the track parameters as the momentum measured by the Muon System or the E/p using the calorimetry information. The alignment chain starts at the trigger level where a stream of high pT and isolated tracks is selected online. Also a cosmic ray trigger is enabled while ATLAS is recording collision data, Thus a stream of cosmic-ray tracks is recorded exactly with the same detector operating conditions as the normal collision tracks. As the alignment algorithms are based on the minimization of the track-hit residuals, one needs to solve a linear system with large number of degrees of freedom. The solving involves the inversion or diagonalization of a large matrix that may be dense. We will present results of the alignment of the ATLAS tracker using the 2011 collision data. The validation of the alignment is performed first using its own observables (track-hit residuals) as well as using many other physics observables. The results of the alignment with real data reveals that the attained precision for the alignment parameters is approximately 5μm.