Alignment of the ATLAS Inner Detector Tracking System

ATLAS is a multi purpose detector built to study proton-proton collisions, at the center of mass energy of 14 TeV, as provided by the Large Hadron Collider (LHC) at CERN. ATLAS is equipped with an inner charged particle tracking system composed of silicon and drift tube based detectors: as referred to a the ATLAS Inner Detector (ID). The required precision for the alignment of the most sensitive coordinates of the silicon sensors is at the level of few microns, the limit being derived by the requirement that module misalignments should not worsen the resolution of the track parameter measurements by more than 20%. The outline of the alignment approaches and results obtained using real data from cosmic rays and large scale computing simulation of physics samples are presented. Cosmic ray data serves to derive an early set of alignment constants for the ATLAS ID before the LHC start up. The impact of the alignment on physics measurements is also discussed.