Track based Alignment of the Inner Detector of ATLAS

ATLAS is a multipurpose experiment that records the LHC collisions. In order to reconstruct trajectories of charged particle, ATLAS is equipped with a tracking system built using different technologies, silicon planar sensors (pixel and microstrips) and drift‐tubes. which is embedded in a 2 T solenoidal field. 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. Primary vertexing and beam spot constraints have also been implemented, as well as constraints from on the particle momentum as measured by the Muon System. Finally the assembly survey data can be used as constraint to the alignment corrections. As alignment algorithms are based on minimization of the track‐hit residuals, one needs to solve a linear system with large number of DoF. The solving involves the inversion or diagonalization of a large matrix that may be dense. The alignment jobs are executed at the CERN Analysis Facility. The event processing is run in parallel in many jobs. The output matrices from all jobs are added before solving. We will present the results of the alignment of the ATLAS tracker using data recorded during 2010 and 2011 using the LHC proton‐proton collision runs at 7 TeV. Validation of the alignment was performed by measuring the alignment observables as well as many other physics observables, notably resonance invariant masses in a wide mass range (K0s, J/ψ and Z decays into μ+μ‐) and the effect of detector systematic distortions on their invariant mass and momentum. Also the E/p for electrons has been studied. The results of the alignment with real data reveal that the precision of the alignment constants is approximately 5 microns.