20-27 July 2011
Alpes Congrès - Alpexpo
Europe/Paris timezone

Alignment of the ATLAS Inner Detector tracking system

Not scheduled
Dauphine (Alpes Congrès - Alpexpo)


Alpes Congrès - Alpexpo

Poster Detector R & D and data handling


Ms Louise Skinnari (UC Berkeley / LBNL)


ATLAS is a multipurpose experiment that records the products of the LHC collisions. In order to reconstruct trajectories of charged particle 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, the alignment of the ATLAS tracking system requires the determine accurately its almost 700,000 degrees of freedom. Thus the demanded precision for the alignment of the silicon sensors is below 10 micrometers. This implies to use a large sample of high momentum and isolated charge particle tracks. The high level trigger selects those tracks online. Then the raw data with the hits information of the triggered tracks is stored in a calibration stream. Tracks from cosmic trigger during empty LHC bunches are also used as input for the alignment. 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 detector using real data recorded during 2010 and 2011 using the 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 μ 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.

Primary author

Naoko Kanaya (ICEPP, University of Tokyo)

Presentation Materials