The Trimmed Kalman vertex fitter

Complete: 5

Overview

Trimmed Kalman vertex fitter (TKVF)is a conventional robustification of the Kalman filter, where tracks incompatible with the vertex are removed one by one from the vertex. It is a hard-assignment, iterative fit.

First, all input tracks are fitted to a vertex. The track least compatible with that vertex is removed, and the vertex is refitted. This procedure is repeated, until the compatibility of all tracks is below a given threshold. Once a track is rejected, it will not be included again in the vertex.

The compatibility of each track to the vertex is computed from the standardized distance to the vertex. For correctly assigned tracks, provided that the track parameter errors are correctly estimated at the track fitting stage, this distance is distributed according to a chi^2 distribution with 2 degrees of freedom (1 degree of freedom for a 2-track vertex). If the above-mentioned threshold is chosen as the (1- alpha) quantile of the chi^2 distribution and if the vertex has no incorrectly assigned tracks, then the probability of rejecting a good track is equal to alpha. The probabilistic meaning of the compatibility cut renders the tuning of the algorithm straightforward. However, if there were incorrectly assigned tracks, the probability of rejecting a good track may be well above alpha.

The fitter is implemented in the package RecoVertex/TrimmedVertexFit

An easy way to use the TKVF is through the TrimmedVertexFitter, which defines reasonable defaults. Although it is a VertexFitter, it can only be called with a vector of TransientTracks :

TrimmedVertexFitter fitter;
TransientVertex myVertex = fitter.vertex(vectorOfRecTracks);

Parameters

To change the parameters, use the appropriate set methods or give a PSet in the constructor. The PSet needed is the following:

GSFParameters = cms.PSet(
        minPt = cms.double(0.),
        trackCompatibilityCut = cms.double(0.01),
        vtxFitProbCut = cms.double(0.01)
    )

The parameters are the following:

Parameter Name Description Default
minPt The pT cut to apply to the tracks 0.
trackCompatibilityCut probability below which a track is considered incompatible with the 1st vertex candidate formed 0.05
vtxFitProbCut probability below which a vertex is rejected 0.01

Review status

Reviewer/Editor and Date Comments
Main.speer -14 Feb 2007 new page
JennyWilliams - 10 Apr 2007 edited for swguide inclusion
ThomasSpeer - 27 Feb 2009 Review and update

Responsible: Main.speer (Thomas Speer)

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Topic revision: r3 - 2009-03-12 - ThomasSpeer
 
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