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Alignment-aware reconstruction

Alignment-aware reconstruction. (see also IB’s talk at Offline Week, 16.03.2007). I. Belikov, C. Cheshkov, A. Dainese, A. Gheata, R. Grosso. Sources of the geometry info in AliRoot. 1) Ideal geometry. Generated in C++ code, based on the technical drawings.

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Alignment-aware reconstruction

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  1. Alignment-aware reconstruction (see also IB’s talk at Offline Week, 16.03.2007) I. Belikov,C. Cheshkov,A. Dainese, A. Gheata, R. Grosso Offline Week, CERN, 9 Jul 2008

  2. Sources of the geometry info in AliRoot 1) Ideal geometry. Generated in C++ code, based on the technical drawings. The results are stored in - geometry.root - $OCDB_PATH/GRP/Geometry/ 2) A set of AliAlignObjects, describing the deviations from the ideal geometry. - Generated by special macros - Coming out of the dedicated alignment procedures (or survey data) The results are stored in - $OCDB_PATH/DET/Align/ Simulation, Reconstruction, Visualization share the same copy of the geometry I. Belikov Offline Week, CERN, 9 Jul 2008 2

  3. AliReconstruction … // Load the ideal geometry (from either local geometry.root or OCDB) … // Load the alignment objects (from ODCB) … // Misalign the loaded ideal geometry. // (the volumes, and so the clusters, moved physically in space !) … // Create the trackers out of the loaded (typically ideal) geometry … for (…) { //**************** Loop over events ****************** … AliTracker *tracker=…; // All the detector specific trackers tracker->LoadClusters(…); // Express (not move !) the clusters … // in the ideal (tracking) coordinate system tracker->Clusters2Tracks(…); // “Two step” tracking … } I. Belikov Offline Week, CERN, 9 Jul 2008 3

  4. Xcl 2 1 track “Two-step” alignment-aware tracking • Inside LoadClusters(…) • Transform the clusters to • the tracking CS (AliCluster) • The clusters do not move ! • Inside Clusters2Tracks(…) • Propagate the track to the • ref. plane Xref (point 1), using the misaligned geometry for the material corrections. • Look around for the • clusters • Propagate the track to the • cluster plane Xcl (point 2) • Update the track params. XZ view X Xref Z • At the moment of the trackers creation, • the misaligned (blue) geometry is already • in memory. • Inside the tracker’s constructors: • Simplify/idealize the geometry (dots) Offline Week, CERN, 9 Jul 2008

  5. Xl Yl Local Coordinate System Definition: the origin is at the center of a module, the axises are parallel to the faces. • Advantages: • The native TGeo CS. • Local corrections are natural… • Disadvantages: • Not optimal • for the tracking; • (Both translations and • rotations are needed • for L -> L’ and L -> G) • “Alignable”does not • always coincide with • “sensitive” Xl Zl Offline Week, CERN, 16 Mar 2007

  6. X’t Y’t Right angle The blue track goes with the transformation T -> T’ that is just a single rotation around Z Tracking Coordinate System Definition: the origin and the Z-axis coincide with the ones of the Global CS, X-axis goes perpendicular to a module face (ideal). t • Disadvantages : • Not a native TGeo LCS. • Advantages: • A“tracking” (plane) can be • defined at the “sensitive”. • Optimal for the tracking: • simplest possible equations, • transformations T <-> T’ and • T <-> G are simple rotations • around the Z-axis. t Offline Week, CERN, 16 Mar 2007

  7. Misalignment and the relations between the CS’s(a bit of the implementation) XY view YZ view Yg Xl Xo Xl Yl Xt Zt Yo Yl Zl Yt • These threeMt, Ml, Momatrices are enough • to make the tracking alignment-aware • to do even more… (see Cvetan’s talk) • In a detector independent way • (AliCluster) Yl Xg Xt Ml(local to global matrix) Transforms the local (misaligned) CS to the global CS Mo(original to global matrix) Transforms the original (ideal) local CS to the global CS (Cvetan, Andrei) Mt (tracking tolocal matrix) Transforms the tracking CS to the local CS : a) Offsets the origin in X (sometimes), Y (sometimes), Z b) Swaps the axes X <-> Y (Ludovic, Andrea) Offline Week, CERN, 16 Mar 2007

  8. Running the simulation/reconstruction • Generate a misalignment file using the macros (MakeITSFullMisAlignment.C, e.g.) and put it to CDB. • Run the simulation in the standard way. • You will get the data produced with the misaligned geometry. • Run the reconstruction in the standard way. • The reconstruction will take into account the alignment info. • Note the results. • Suppress reading the alignment info and re-run the reconstruction: AliReconstruction rec; rec.SetLoadAlignData(“”); rec.Run(); • Compare these results with the realigned ones. Offline Week, CERN, 16 Mar 2007

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