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A simplified geometry for track fitting  
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A solution to this problem is to use a 'simplified' tracking geometry. In LHCb we have for now chosen a very simplistic solution: the detector geometry is 'summarized' in O(20) volumes, most of which are in the velo region. These volumes get material properties such that they more or less represent the average material that you would find in the full geometry in that volume.  
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> >  Here you can find a few presentations in which the simplified geometry is discussed  
Volumes in the simplified geometry  
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> >  The tracking geometry contains about 30 volumes. The xml for the geometry resides in the database in DDDB/TrackfitGeometry/. It is a normal geometry, just like the default LHCb geometry. You can for example visualize it in Panoramix.
Here are a few schematic pictures of how the volumes are chosen:
To get access to the toplevel element of the simplified geometry in C++ you can do
Each volume in the trackfit geometry is either a box, a cylinder or a cone. Being a single volume, it has a homogeneous material distribution. The volumes in the track fit geometry have been defined by carefully looking at how material was distributed. The aim was to obtain pull distributions for the parameters of long tracks that at the vertex look more or less identical to those that are obtained with the nominal geometry. The particular complication is the RF foil: the simplified geometry is not phi symmetric, just like the full LHCb geometry is not phi symmetric. The material properties of the volumes in the trackfit geometry are obtained by averaging the true distribution of materials inside the volume. This procedure is described below.
Use of the tracking geometry in the trackfit
The track fit uses a tool of type
Fitter.addTool( DetailedMaterialLocator , "MaterialLocator" ) Fitter.MaterialLocator.Geometry = '/dd/TrackfitGeometry/Structure/LHCb'
Alternatively, you can use another implementation of the
Fitter.addTool( SimplifiedMaterialLocator , "MaterialLocator" )
The latter is a bit faster because the  
Procedure to generate the material properties of the volumes  
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> >  The package Det/TrackfitGeometry contains an algorithm to generate the material properties of the algorithms in the simplified geometry. The procedure can be sketched as follows:
To run the algorithm, checkout the package, compile the algorithm and run the example script: shell> SetupProject Brunel shell> getpack Det/TrackfitGeometry shell> cd Det/TrackfitGeometry/cmt shell> gmake shell> gaudirun.py ../options/GenerateMaterial.pyThis will produce a file materials.xml as well as a root file with histograms.
To test your new shell> mkdirhier DDDB/TrackingGeometry shell> mv materials.xml DDDB/TrackingGeometry shell> SetupProject LHCb shell> copy_files_to_db.py c sqlite_file:SimplifiedMaterials.db/DDDB s DDDBYou can load this database layer in your standard brunel job by copying these lines into your main option file shell> from Configurables import ( CondDB, CondDBAccessSvc ) shell> simplifiedmaterialCond = CondDBAccessSvc( 'SimplifiedMaterialCond' ) shell> simplifiedmaterialCond.ConnectionString = 'sqlite_file:SimplifiedMaterials.db/DDDB' shell> CondDB().addLayer( simplifiedmaterialCond )
 
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A simplified geometry for track fitting
Introduction  
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< <  To correct for energy loss and multiple scattering on a charged particle trajectory, the LHCb track fit talks to the transport service to locate intersections with detector material on the track.  
> >  To correct for energy loss and multiple scattering on a charged particle trajectory, the LHCb track fit talks to the transport service to locate intersections with detector material. Since the number of volumes in the LHCb geometry is large (about 10 million), searching for intersections is expensive. For single iteration fits, it dominates the track fit time consumption.
A solution to this problem is to use a 'simplified' tracking geometry. In LHCb we have for now chosen a very simplistic solution: the detector geometry is 'summarized' in O(20) volumes, most of which are in the velo region. These volumes get material properties such that they more or less represent the average material that you would find in the full geometry in that volume.  
Volumes in the simplified geometry 
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A simplified geometry for track fitting
IntroductionTo correct for energy loss and multiple scattering on a charged particle trajectory, the LHCb track fit talks to the transport service to locate intersections with detector material on the track.
Volumes in the simplified geometry
Procedure to generate the material properties of the volumes
 WouterHulsbergen  14 Jul 2009 