Alignment of the ATLAS Inner Detector Tracking System

Introduction The ATLAS Inner Detector (ID) consists of two silicon subsystems, the Pixel detector and the Semiconductor Tracker (SCT), complemented by the Transition Radiation Tracker (TRT) composed of drift tubes. After the assembly of the detector, the position of the individual modules is known with much worse accuracy than their intrinsic resolution. Therefore a track-based alignment procedure has to be applied. The baseline goal of the alignment is to determine the position and orientation of the modules with such precision that the track parameters' determination is not worsened by more than 20% with respect to that expected from the perfectly aligned detector. This is crucial for efficient track reconstruction and precise momentum measurement and vertex reconstruction.