7.3.1 Offline Primary Vertex Finding

Contents

• Primary Vertex reconstruction
• Vertex reconstruction analysis
• Review status

Primary Vertex reconstruction

The reconstruction of Primary vertices using the full tracks is part of the default reconstruction sequence. It is done using the generalTracks track collection. The vertices are stored in the event as reco::Vertex collections. Two collections are stored in the event:

• offlinePrimaryVertices : The standard vertex collection, where the offlineBeamSpot is used to filter the tracks. This is the collection which we advise to use.
• offlinePrimaryVerticesWithBS : For this collection, the offlineBeamSpot is used not only to filter the tracks, but also as an additional constraint in the fit. It improves the resolution of the vertices, but we do not yet advise to use it until further robustness studies are done, especially for the initial conditions.

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. More information on how to redo the primary vertex reconstruction is given in the offline guide.

From CMSSW 1.8.0 onwards, if no reconstructed vertex is found in an event using the tracks, a vertex based on the [[SWGuideFindingBeamSpot][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.

Vertex reconstruction analysis

In cmsRun mode, the primary vertices can be analyzed with the CMS.PrimaryVertexAnalyzer module by running demoAnalyzePrimaryVertex_cfg.py:

cmsRun demoAnalyzePrimaryVertex_cfg.py

in the same directory. A root histogram file simpleVertexAnalyzer.root is produced, that, in addition to histograms of the fitted position, covariance, chi2 and degrees of freedom, also contains results of sanity checks of the number of valid track links and invalid fit parameters.

root simpleVertexAnalyzer.root
pullx->Draw()
eff->Draw()
nans->Draw()

A simple test of the input track parameters is done by

cmsRun demoAnalyzeTracks_cfg.py

which produces histograms of track parameter pulls. For instance, to look at curvature and z0:

root simpleTrackAnalyzer.root
pull0->Draw()
pull4->Draw()

Review status

Responsible: ThomasSpeer
Last reviewed by: ThomasSpeer - 16 Sep 2008