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The task of Vertex Fitting is, given a set of tracks, to compute the best estimate of the vertex parameters (position, covariance matrix, constrained track parameters and their covariances) as well as indicators of the success of the fit (total chi^2, number of degrees of freedom, track weights).

**Contact persons**: T.Speer, W.Waltenberger

The fitters can be used easily via the ConfigurableVertexFitter.

A simple example of the use of a VertexFitter is given on the WorkBook page. A more detailed description is given here.

Vertex fitting algorithms can be divided into least-squares algorithms and robust algorithms. In least-squares algorithms all tracks are used, with a weight 1. The KalmanVertexFitter is such a fitter. Robust fitters are able to downweight tracks, and are thus less sensitive to outliers (mismeasured tracks (type 1 outliers) or tracks from another vertex (type 2 outliers)). A common measure of the of the robustness is the break-down point, i.e. the fraction of outliers below which the vertex fit is not affected.

The fitters now available are:

- The KalmanVertexFitter: the simple least-squares algorithm
- The AdaptiveVertexFitter: iterative re-weighted KalmanFitter which down-weights tracks according to their distance to the vertex
- The TrimmedVertexFitter: conventional robust version of the Kalman fitter, which removes tracks incompatible with the vertex
- The GaussianSumFitter: fitter using the non-Gaussian distributions of measurement errors
- The AdaptiveGsfVertexFitter: a combination of the adaptive fitter and the Gaussian-sum fitter

After a vertex fit, the track parameters can optionally be re-estimated at the fitted vertex position, using this position as a constraint. Usage and examples are given on this page.

The classes SimpleVertexTree is provided to produce a simple TTree to analyse the results of a fitter

Reviewer/Editor and Date (copy from screen) | Comments |
---|---|

Main.speer -17 Jul 2006 | new page (Thomas Speer) |

JennyWilliams - 28 Mar 2007 | edited for reorganisation of swguide vertexing pages |

Responsible: Main.speer

Last reviewed by: Most recent reviewer

This topic: CMSPublic > CMSCommunicationsGroup > CMSCommunicationsProjects > WebHome > SWGuide > SWGuideVertexReco > SWGuideVertexFitting

Topic revision: r19 - 2009-02-27 - ThomasSpeer

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