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Validation and Implementation of ID Alignment Strategies in the ATLAS Experiment

The ATLAS CSC was pivotal in validating ID alignment algorithms through extensive Monte Carlo simulations. Our group contributed to the global χ² silicon alignment, which utilized track χ² minimization and involved inverting N×A matrix dimensions up to 35,000. The alignment process was systematically executed, starting from large structures to individual modules. Results of the global χ² method were incorporated into the ATLAS detector paper. Notably, during the 2008 ATLAS Full Dress Rehearsals, we developed and validated alignment constants, which facilitated the alignment of the Inner Detector using real cosmic ray data.

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Validation and Implementation of ID Alignment Strategies in the ATLAS Experiment

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  1. ID Alignment: CSC • The ATLAS CSC (Computing System Commissioning) served to validate the ATLAS ID Alignment algorithms with large MC samples • Our group participates in the Globalχ2 silicon alignment based on the tracks χ2 minimization • Needs to invert (diagonalize) a NAlixNAli matrix (up to 35k x 35k)

  2. ID Alignment: CSC • During the CSC the ID alignment strategy was settle level by level • From big structures (few DoFs) • Barrel layers and end caps disks (hundreds of DoFs) • To modules (35K DoFs)

  3. ID Alignment: CSC • Results of the Globalχ2 method during the CSC were used in the ATLAS detector paper • The ATLAS Collaboration, G. Aad et al., • The ATLAS Experiment at the CERN Large Hadron Collider, • JINST 3 (2008) S08003

  4. ID Alignment operation scheme

  5. ID Alignment FDR • The 2008 ATLAS Full Dress Rehearsals main purpose was an evaluation of different data streaming techniques for physics analysis and calibration • The calibration and alignment was run like in real data taking with a 24 hours loop • We participated actively in all FDR exercises • Running the alignment jobs • Producing alignment constants • Reporting at the daily operations meetings • Validating the alignment constants • Our alignment constants were used for the official production

  6. ID Alignment FDR • During the 2008 ATLAS FDRs the alignment scheme was implemented, tested and validated. The Ringberg ID alignment scheme

  7. ID Alignment with real cosmic data 2008 • During the fall of 2008, ATLAS collected several millions of real cosmic ray tracks. • They served to commission the real detector • Align the real Inner Detector with real data

  8. ID Alignment with real cosmic data 2008 • Our first set of alignment constants with the Globalχ2 was validated 18th /September/2008 and it was ready to reconstruct the first LHC collisions expected the following day • The experience with the real data helped to identify new alignment levels (not anticipated in the simulation) to account for collective movements of modules during the assembly • Pixel half shells • Pixel and SCT ladders

  9. ID Alignment with real cosmic data 2008 • Our Globalχ2 set of constants are used for the • Official ATLAS cosmic data reprocessing • By the Cosmic analysis group • Validation performed with track splitting in upper and lower segment and check the matching

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