Alignment of the ATLAS Inner Detector Tracking System

ATLAS is one of four multipurpose experiments that records the products of the LHC proton-proton 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 solenoidal 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 almost 36,000 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 resulting required precision for the alignment is below 10 μm for the pixel modules, 20 μm for the microstrip modules, and about 130 μm for the drift-tube based detector modules. 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. We will present the status and performance of the ATLAS Inner Detector alignment used for 2011 physics analyses. Validation of the alignment is performed by measuring alignment observables. Results based on 7 TeV collisions data, recorded during 2010 will be shown and compared to expectations from Monte Carlo simulated data assuming a perfectly aligned detector.