A simplified geometry for track fitting


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.

Here you can find a few presentations in which the simplified geometry is discussed

Volumes in the simplified geometry

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 top-level element of the simplified geometry in C++ you can do IDetectorElement* simplifiedgeometry = getDet("/dd/TrackfitGeometry/Structure/LHCb" ) ;

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 IMaterialLocator to interact with the geometry. There are two ways to use the simplified geometry in the track fit. First, you can tell the DetailedMaterialLocator to use a different geometry to search for volumes, e.g. schematically

    Fitter.addTool( DetailedMaterialLocator , "MaterialLocator" )
    Fitter.MaterialLocator.Geometry = '/dd/TrackfitGeometry/Structure/LHCb'

Alternatively, you can use another implementation of the !IMaterialLocator tool:

    Fitter.addTool( SimplifiedMaterialLocator , "MaterialLocator" )

The latter is a bit faster because the SimplifiedMaterialLocator exploits a particular feature of the simplified geometry: None of the volumes in the simplified geometry are rotated. Therefore, the global-to-local transformations are much simpler than for the generic case.

Procedure to generate the material properties of the volumes

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:

  • take a DST with simulated data that contains both MCParticles and =MCHits=. (For MC09 this is an '=.xdst=' file.)
  • for each event, select MCParticles that traverse both VELO and T-stations.
  • make straight-line segments between any two MCHits and between the first MCHit and the vertex
  • for each segment use TransportSvc to compute the intersections with the full geometry. An intersection is a special term int he transport service: it is basically the segment through as single-material volume. So, you have a begin-point, an end-point and a pointer to a particular material in the material database
  • for each of these intersections, use the 'simplified' TransportSvc (which is a transport svc using the simplified geometry) to compute the corresponding volumes in the implified geometry. this allows to divide the original intersection precisely over the volumes in the simplified geometry.
  • in this fashion, accumulate for each volume a list of 'Materials' that have been seen. Keep for each material the 'total segment length'. The latter represents the volume fraction of that material in the volume of the simplified material.
  • when all events have been processed, the materials for each volume are combined in a Mixture (which is an implementation of a Material). To do this the original Materials are decomposed in so called Elements (which is another implementation of a Material). Each element appears in the final mixture as a certain fraction of the mass. This allows to get proper values for the effective A and Z for the mixture. The density and radiation length of the material follow their own averaging:
    • the density is the volume weighted density of all materials
    • the inverse radiation length is the volume weighted inverse radiation length. This assures that the average is consistent with the way it is used in the computation of the multiple scattering contribution used in the track fit

To run the algorithm, check-out 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.py
This will produce a file materials.xml as well as a root file with histograms.

To test your new material.xml file, you can create a database layer as follows:

   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 DDDB
You 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 )

-- WouterHulsbergen - 14 Jul 2009

Topic attachments
I Attachment History Action Size Date Who Comment
PNGpng rffoil.png r1 manage 38.6 K 2009-07-14 - 15:54 WouterHulsbergen xy view of velo with RF foil in red
PNGpng simplelhcb.png r1 manage 6.5 K 2009-07-14 - 15:52 WouterHulsbergen all of LHCb
PNGpng veloregion.png r1 manage 7.3 K 2009-07-14 - 15:53 WouterHulsbergen velo region
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Topic revision: r4 - 2016-08-24 - GeorgeRufusCoombs
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