THE ATLAS INNER DETECTOR TRACK BASED ALIGNMENT

The alignment of the ATLAS Inner Detector is performed with a track-based alignment algorithm. Its goal is to provide an accurate description of the detector geometry such that track parameters are accurately determined and free from biases. Its software implementation is modular and configurable, with a clear separation of the alignment algorithm from the detector system specifics and the database handling. The alignment must cope with the rapid movements of the detector as well as with the slow drift of the different mechanical units. Prompt alignment constants are derived for every run at the calibration stage. These sets of constants are then dynamically split from the beginning of the run in many chunks, allowing to describe the tracker geometry as it evolves with time. The alignment of the Inner Detector is validated and improved by studying resonance decays (Z and J/psi to mu+mu-), as well as using information from the calorimeter system with the E/p method with electrons. A detailed study of these resonances (together with the prpperties of their decay products) allows correcting for alignment weak modes such as detector curls, twists or radial deformations that may bias the momentum and/or the impact parameters. On the other hand, the detailed scrutiny of the track-hit residuals serves to asses the shape of the Pixels and IBL modules.