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Revision 392008-02-11 - ThomasSpeer

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META TOPICPARENT name="WorkBookVertexReco"

7.4.1 Offline Primary Vertex Finding

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Contents

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Algorithm description

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Primary Vertex reconstruction

 
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The offline primary vertex finder algorithm is documented in CMS Note 2006/029, section 3.1. In short, it proceeds as follows:
  • the reconstructed tracks are clustered into several primary vertex candidates, according to the z-coordinate of the point of closest approach of the tracks to the z-axis
  • each primary vertex candidate is cleaned up from incompatible (secondary) tracks
  • primary vertex candidates compatible with the beam line are retained.
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The reconstruction of Primary vertices using the full tracks is part of the default reconstruction sequence. It is done once on CKF and once on RS tracks. More information on how to redo the primary vertex reconstruction is given in the offline guide. The vertices are stored in the event as reco::Vertex collections, with names offlinePrimaryVerticesFroCFKTracks and offlinePrimaryVerticesFromRSTracks. Opening a data file in root, the data of the primary vertices (such as the coordinates, chi-squared, degrees of freedom) can be viewed direclty.
 
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Instructions for CMSSW_1_2_0_pre9

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From CMSSW 1.8.0 onwards, if no reconstructed vertex is found in an event using the tracks, a vertex based on the [[WorkBookFindingBeamSpot][beam-spot] is put into the event. In this case, the vertex contains no tracks (as non have been used) , the chi^2 and the ndof are 0, and the flag isFake() is set to true.
 
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Production of primary vertices

Offline primary vertices can be reconstructed from RS or CKF tracks. Two different modules, OfflinePrimaryVerticesFromRSTracks and OfflinePrimaryVerticesFromCTFTracks are thus provided. The corresponding default config files can be found in cvs in

RecoVertex/PrimaryVertexProducer/data/OfflinePrimaryVerticesFromRSTracks.cfi, RecoVertex/PrimaryVertexProducer/data/OfflinePrimaryVerticesFromCTFTracks.cfi.
The sequences vertexreco (for CTF trakcs) and vertexreco_rs (for RS tracks) are defined in RecoVertex/Configuration/data/RecoVertex.cff and putting either one in the path decides about the type of tracks to be used.

The example below shows how to reconstruct primary vertices from a file containing tracks, and store these vertices in a file called pv_reco.root. The module sequence is defined in the following CMSSW config file demoProducePrimaryVertex.cfg. The tracks used on input are also copied, for analysis and debugging.

scramv1 project CMSSW CMSSW_1_2_0_pre9
cd CMSSW_1_2_0_pre9/src
project CMSSW
cvs co -r V00-01-10 Validation/RecoVertex
cd Validation/RecoVertex
scramv1 b
cd test
eval `scramv1 runtime -csh`
cmsRun demoProducePrimaryVertex.cfg

This produces pv_reco.root. To see it:

root -l pv_reco.root
TBrowser a;

This brings up the ROOT browser. Click on ROOT Files and on pv_reco.root to access the event data. Click on Events. The data collections retained, tracks and vertices, now appear. Clicking further, you may have a quick look at the distributions of the x, y, z, chi-squared, degrees of freedom of the reconstructed vertices

To exit ROOT, enter:
.q

 
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Vertex reconstruction analysis

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Vertex reconstruction analysis

  In cmsRun mode, the primary vertices can be analyzed with the PrimaryVertexAnalyzer module by running demoAnalyzePrimaryVertex.cfg:
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Reviewer/Editor and Date (copy from screen) Comments
Main.werdmann - 04 Dec 2006 update
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ThomasSpeer - 11 Feb 2008 update
  Responsible: ThomasSpeer
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Last reviewed by: Werdmann - 04 Dec 2006
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Last reviewed by: ThomasSpeer - 11 Feb 2008
 
META FILEATTACHMENT attr="" autoattached="1" comment="" date="1154101484" name="analyzePrimaryVertex_cfg.txt" path="analyzePrimaryVertex_cfg.txt" size="437" user="Main.AnneHeavey" version="1"
META FILEATTACHMENT attr="" autoattached="1" comment="" date="1150475347" name="pv.C" path="pv.C" size="413" user="vanlaer" version="1.1"
 
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