Alignment of the ATLAS Inner Detector

Atlas is a multipurpose experiment that records the LHC collisions. In order to reconstruct the trajectories of charged particles, ATLAS is equipped with a tracking system built using distinct technologies: silicon planar sensors (both pixel and microstrips) and drift-tubes. The tracking system is embedded in a 2 T solenoidal field. In order to reach the track parameter accuracy requested by the physics goals of the experiment, the ATLAS tracking system requires to determine accurately its almost 700,000 degrees of freedom. The demanded precision for the alignment of the silicon sensors is below 10 micrometers. The implementation of the track based alignment within the ATLAS software framework unifies different alignment approaches and allows the alignment of all tracking subsystems together. The alignment software counts of course on the tracking information (track-hit residuals) but also includes the capability to set constraints on the beam spot and primary vertex for the global positioning, plus constraints on the track parameters as the momentum measured by the Muon System or the E/p using the calorimetry information. We will present results of the alignment of the ATLAS tracker using proton-proton collision data recorded during the 2011 LHC run. The detector stability over large periods will be also evaluated. The validation of the alignment is performed first using its own observables (track-hit residuals) as well as using many other physics observables, notably the resonance invariant masses in a wide energy range (K0s, J/ψ and Z decays in to μ+μ-) and the effect of the detector systematic distortions (curl, twist, telescope, etc) on the reconstructed invariant mass and on the μ momentum. Also the electrons E/p has been studied mainly in the W→eν channel. The results of the alignment with real data reveals that the attained precision for the alignment parameters is approximately 5 micrometer.