Alignment of the ATLAS Inner Detector Tracking System

ATLAS is one of the multipurpose experiments that records the products of the LHC proton-proton and heavy ion collisions. In order to reconstruct trajectories of charged particles produced in these collisions, ATLAS is equipped with a tracking system built using two different technologies, silicon planar sensors (pixel and microstrips) and drift-tube based detectors. Together they constitute the ATLAS Inner Detector, which is embedded in a 2 T axial field. Efficiently reconstructing tracks from charged particles traversing the detector, and precisely measure their momenta is of crucial importance for physics analyses. In order to achieve its scientific goals, an alignment of the ATLAS Inner Detector is required to accurately determine its more than 700,000 degrees of freedom. The goal of the alignment is set such that the limited knowledge of the sensor locations should not deteriorate the resolution of track parameters by more than 20% with respect to the intrinsic tracker resolution. The implementation of the track based alignment unifies different approaches which allow for aligning all tracking subsystems together. The alignment procedure is based on minimization of track-hit residuals, the distance between the extrapolated track position on a given detector module to the position of the recorded hit in the same module, and involves solving a linear system with a large number of degrees of freedom. The alignment is performed using a combination of isolated high transverse momentum tracks from 7 TeV collisions at the LHC, and cosmic ray tracks. The performance of the ATLAS Inner Detector alignment used for 2011 physics analyses is presented. Results based on 7 TeV collisions data, recorded during 2010 are shown and compared to expectations from Monte Carlo simulated data assuming a perfectly aligned detector.